Notes on Microbial Infection for Medical Physicists

Microbes are responsible for some very unpleasant conditions. Some infections become apparent very quickly. Bacterial meningitis caused by Neisseria meningitidis can lead to death within hours of the first symptoms. Others may take many years to become apparent, even though infected individuals may pass on their infection to others. One example is HIV infection and AIDS. Another is the hepatitis B virus, which is associated with the development of hepatocarcinoma many years after primary infection. Furthermore, individuals may harbour infections without showing any sign of disease. These are so-called 'carriers'. A famous case was Typhoid Mary - a cook who infected the families she worked for, until she was pensioned off. Blood donors are screened for infections including hepatitis B, HIV and syphilis. Organ transplant donors may pass on cytomegalovirus to susceptible recipients.

Because of the risk of contracting infections from samples provided by carriers, all clinical specimens should be regarded as contaminated. Specimens from individuals who are known to be infected with particular microbes may be labelled to indicate that they are a 'high risk'. Paradoxically, it is the unidentified carrier that poses the greater risk.

Long experience has allowed us to assess the degree of risk individual microbes pose to human health. Disease-causing microbes are referred to as pathogens and pathogens may be categorised with respect to the degree of hazard that they pose to human health.

• Group 1 are the safest and are considered non-pathogenic.
  
  
• Group 2 pathogens pose a slight risk of infection. They are easily contained using 'good microbiological practice' Many microbes fall into Group 2.
  
  
• Group 3 pathogens are highly infectious. These include such microbes as Salmonella typhi and Mycobacterium tuberculosis. Group 3 pathogens require special containment facilities.
  
  
• Group 4 pathogens are the most dangerous. There are relatively few Group 4 pathogens, mostly viruses - examples being the virus that causes Lassa fever and the smallpox virus. These require containment facilities that are only found in specialist establishments.

Before working in particular environments, vaccination may be offered. With tuberculosis, the immune status is checked before offering vaccination. Active immunisation is preferable. This may either be with a killed vaccine or a live microbe that has lost its virulence - known as an attenuated strain. A good example of an attenuated vaccine is that used for poliomyelitis. Alternatively, non-pathogenic relatives that share an antigenic structure with major pathogens may be used to induce immunity. Examples include the vaccina virus, historically used to immunise against smallpox and the BCG vaccine for tuberculosis. BCG stands for Bacille Calmette Guerrin - an avirulent close relative of Mycobacterium tuberculosis.

A recent development is the sub-unit vaccine. A virus sub-unit vaccine contains the antigenic structure of the virus from which protection is sought, but is lacking in nucleic acid. It is incapable of infection or replication. If an individual has been exposed to a particularly virulent infectious agent, then it may be possible to provide passive immunity. To do so, antiserum from patients who have survived the infection may be given. The antibodies in the serum will offer a degree of protection during the period just following exposure. Because of the risk of blood-borne infections, this is reserved for the most serious of life-threatening infections.

 Protective clothing should be worn in high risk areas. It must be fastened properly. An open lab coat protects only the wearer's back. White laboratory overalls show dirt more easily than coloured garments, and this prompts laboratory workers to change coats frequently. Cuts and grazes should be covered with suitable waterproof dressings. Gloves, masks and goggles may be used as appropriate. Viruses such as hepatitis B and bacteria such as Neisseria gonorrhoeae may infect through splashes in the eye. Try explaining that to a partner!

The word 'pathogen' is derived from the Greek word 'pathos', meaning suffering. The vast majority of microbes in the world are not pathogenic, and may, indeed be beneficial to life. Medical microbiology is concerned with the small minority of microbes that are pathogens, and thus by definition are capable of causing disease. Some pathogens cause trivial and almost inapparent infections. Others are the cause of infections that are life-threatening. These may be rapidly fatal in some cases while in other instances a chronic infection may lead relentlessly to a slow death. Consider meningococcal meningitis that can kill in a matter of hours and compare its course with AIDS.

The ability of a microbe to cause disease is described as its pathogenicty. If this capacity is measurable, some authors will refer to it as virulence, while others use the two terms interchangeably. The word virulence has its roots in the Latin word virus, meaning a poison (of any kind).

When microbiologists talk about viruses, they seem to forget that what they are discussing was at one time described as filterable viruses. This term was coined before the particulate nature of viruses as we know understand them was recognised.

For a microbe to establish an infection, it must first encounter a host. Having done this, it must penetrate its new host to some degree. It then replicates and exits from the host in search of a new victim. This is known as the cycle of infection. If the cycle of infection can be broken, then the pathogen can be contained. Disease is usually the consequence of replication or expulsion of the pathogen.

A few microbes can cause disease without penetration of the host by live pathogens. These are responsible for intoxications. These can range from trivial self-limiting conditions such as staphylococcal food poisoning, which generally resolves within a day, through to life-threatening illnesses like botulism and the mycotoxicoses (the intoxications caused by fungi). Important mycotoxicoses include aflatoxin poisoning, with its associated risk of liver cancer, and ergotism. This is associated with consumption of mouldy rye bread and leads to hallucinatory conditions. LSD is a close molecular relative of ergot.

Some pathogens cause disease by damaging body surfaces. It may be thought that because they do not penetrate the body, the consequences of such infections would not be very severe. In the case of the viruses that cause the common cold this is true. It is not, however, safe to generalise. After all, botulism can be fatal without humans needing to consume live bacteria. In cholera, the gut of the victim is infected by very large numbers of Vibrio cholerae. These bacteria produce a powerful enterotoxin that causes sodium ions to be pumped out from the cells of the gut epithelium into the lumen of the gut. This causes the loss of body fluids by the physical process of osmosis. Victims die of fluid loss and electrolyte imbalance. At post-mortem, the gut appears undamaged, even at the microscopic level.

There are pathogens that have evolved to damage the surfaces that they infect. These include Shigella dysenteriae, the cause of bacillary dysentery. This is an infection of the intestine where the pathogens penetrate into the sub-mucosal layer but go no further. It causes ulceration of the gut and intestinal bleeding and has been called the bloody flux because of the nature of the diarrhoea it causes.

Lastly, there are pathogens that have evolved to live within humans, causing often very severe disease. An important example is typhoid. Salmonella typhi, the bacterium that causes this infection, actually lives within the phagocytic white blood cells that normally defend us from infection.

The human immunodeficiency virus has also evolved to live inside white blood cells that are part of our immune defences. Pathogens that penetrate into the body almost all do severe damage to the victim of infection but this, again is not always the case.

There are a number of viruses that cause latent infections. Examples belong to the herpesvirus family. The viruses responsible for cold sores or genital herpes, herpes simplex viruses 1 and 2, may cause an inapparent first infection. The virus then infects nerve cells and the infection becomes latent. During a latent infection, no virus particles can be found in the infected host but virus DNA can be detected in infected nerve cells. Over 80% of the world population carry herpes simplex virus DNA sequences in the trigeminal nerve. Under appropriate conditions (immunosuppression, stress, menstruation, etc.) the herpes virus infection reactivates. Infection spreads down the nerve and causes typical herpetic skin lesions.

Occasionally, scientists have the opportunity to measure virulence in humans. One tragic case led to a greater understanding of host-pathogen relationships. Tuberculosis was once widely known as the "white plague" because of the 'pale and interesting' look it caused in its victims as they became progressively unwell. It was so prevalent in Victorian times that it became the focus of great literature and opera. Alexandre Dumas wrote La Dame aux Caméllias about Violetta, a courtesan dying of tuberculosis. Verdi set this to music in La Traviata. In real life, TB claimed the lives of artists including John Keates, Frederick Chopin and George Orwell.

There was enormous relief when a vaccine for tuberculosis was developed by the great Microbiologist, Robert Koch. In Lübeck in Germany in 1926, however, 249 children were inoculated with a virulent strain of Mycobacterium tuberculosis instead of the intended vaccine strain. Of the 249 children involved in this tragedy, 76 died of tuberculosis. The remainder developed only minor lesions. Each child was exposed to a standard inoculum. Consequently, the different responses of the children to the vaccine reflect differences in the host response to infection since differences between individual children were the only variables in this incident. These differences dictated whether children lived or died.

The Lübeck disaster showed that even the pathogens that cause life-threatening disease cause a spectrum of disease. The same is also true of typhoid. Although untreated with antibiotics a significant number of typhoid victims will die, some people may be infected without realising it. Indeed, some people are carriers of this potentially deadly bacterium. They are colonised and can act a source of infection for others but they do not suffer any ill effects from harbouring the pathogen.

One of the most famous cases of a typhoid carrier was "Typhoid Mary", who was employed as a cook by several families, members of which subsequently developed symptomatic typhoid. A less well known example involves a navvy who worked on the Croydon water system during in the 1930's. He was employed to lay the clean water pipes, but he was also a typhoid carrier who shed bacteria in his urine. When Nature called, he was in the habit of relieving his bladder on the work-site. In this manner he managed to infect the Croydon water supply. This resulted in 310 cases of typhoid and 43 deaths from the disease.

The bacterium Salmonella typhi, the cause of typhoid, is not the only microorganism that causes a complete spectrum of conditions. Approximately 20% of the population carry Streptococcus pneumoniae as part of their respiratory microbiota without ill effect. This pathogen is responsible for acute lobar pneumonia in vulnerable individuals. This infection was at one time popularly known as the "old man's friend" because it caused a mercifully quick death at the end of a painful terminal illness. A similar number of people carry Neisseria meningitidis, the cause of meningococcal meningitis. In overcrowded conditions such as universities, school dormitories or military barracks the carriage rate for this pathogen may rise to close to 100%.

It is not only bacteria that cause asymptomatic infection. The surface antigen of the hepatitis B virus was at one time known as the Australia antigen. This dates from a study of the genetics of human blood types. Haemophiliacs receive multiple blood transfusions and consequently make antibodies to a number of minor antigens in blood. Their sera were thus used in this study and it was found that large numbers of sera reacted strongly with an antigen in the blood from Aboriginal Australians. Haemophiliacs at the time were also at increased risk of developing hepatitis B from their repeated blood transfusion. This was at a time when the association between blood transfusion and hepatitis B was established. We now know that Aboriginal Australians are very likely to be asymptomatic carriers of this virus.

Infections may be transmitted in a number of ways. These include from person-to-person, air-borne, water-borne, food-borne and insect-borne infection. Infections that are food-borne are more generally known as food poisonings.

The Water-borne infections classically include diseases such as typhoid, dysentery and cholera. Not all water-borne diseases are the same, however. Legionella pneumophila, the cause of Legionnaire's disease, for example, may be spread in air-borne water droplets. These may be shed from contaminated showerheads or water-cooled air conditioning systems that have not been adequately maintained.

Person-to-person spread can occur through several routes.

There is an old saying: "coughs and sneezes spread diseases". Many respiratory and systemic infections are spread by the inhalation of infectious droplets.

A liquid medium in which gastrointestinal pathogens may easily spread by the faecal-oral route is provided by diarrhoea.

Some of the most vulnerable of human pathogens, those causing venereal diseases, have evolved to spread during sexual intercourse at moments where humans share a high degree of intimacy. This allows the pathogens to pass between individuals without being exposed to the outside world.

Occasionally, person-to-person infections may spread by direct inoculation. This can occur when, for example, a person is bitten.

Some Animals (including invertebrates) can also cause infection by direct inoculation through bites. Historically, the most important insect-borne infection is the plague. Today, malaria, dengue and yellow fever provide other important examples. Alternatively, direct inoculation can result when contaminated needles are used. Intravenous drug users who share needles with one another or individuals who may be exposed to contaminated needles, for example in tattoo parlours, acupuncture clinics and some hospitals are at particular risk.

Most infections that spread from one individual to another are said to spread horizontally, through a population. When, however, a mother infects her baby the infection is said to be spread vertically. Such transmission is from one generation to the next. Vertical transmission of infection can occur during the development of the foetus, at birth or through infected breast milk.

We are only 10% human. We live in intimate association with a vast number and range of microbes that constitute our commensal microbiota. The commensal microbiota is an important source of infection. Those infections caused by microbes derived from our commensal microbiota are referred to as endogenous infections. These may range from trivial conditions such as boils through to life-threatening infections. An example of an endogenous life-threatening infection is bacterial endocarditis. This is a condition where a-haemolytic streptococci from the mouth or coagulase-negative staphylococci from the skin gain access to the bloodstream and infect the interior of the heart. Endocarditis usually requires previous damage to the heart valves before the pathogens can establish an active infection.

A good example of an endogenoius infection is cystitis. The bacteria responsible are usually found in the bowel or on the skin. Simple anatomy explains why women are much more prone to cystitis than men. Urinary catheterisation also carries a higher than normal risk of Gram-negative septicaemia, which can be fatal. Again the bacteria responsible for this condition are to be found as part of our normal bowel microbiota. Other infections, those contracted from sources outside one's own body, are referred to as exogenous. These may be contracted from contact with another person or from an inanimate object, known as a fomite.

Microbes that are capable of causing overt disease in any susceptible individuals are known as primary pathogens, such as those causing infections like typhoid, cholera and tuberculosis. In contrast, opportunist pathogens only cause infections in people who are to some degree compromised. These compromises may be breakdown in or breach of anatomical structures or may be due to suppression of our immune defences.

The ascendance of AIDS, for example, has illustrated the huge role that opportunist infections can play. The transition from HIV positive to full-blown AIDS is frequently marked by the appearance of one or more chronic opportunist infections such as chronic cryptosporidial diarrhoea or Pneumocystis jiroveci pneumonia. These microbes do not cause overt disease in healthy individuals. Both conditions mark the transition to full-blown AIDS. Opportunist infections often occur when commensal organisms start to cause active disease.

Infections have been likened to continual warfare between the host on one side and pathogens on the other. Primary pathogens are the professional volunteer armies. In contrast, opportunist pathogens are much more like mercenary snipers, working only when conditions are favourable to them.

About one hospital patient in every ten acquires an infection as a direct result of their stay in hospital. Such infections are referred to as nosocomial infections. This term is derived from the Greek word nosos, meaning disease. Hospitalised patients are much more likely to suffer breaches of our natural anatomical defences. It has been said that every surgical operation is an experiment in bacteriology. There is also a risk posed by surgical techniques. 'Clean' wounds in areas where there is little or no commensal microbiota are unlikely to be contaminated but a significant minority of bowel operations do become infected.

It should be noted that the reason that surgical gloves are now worn was not, when the practice was first introduced, because surgeons wanted to prevent contamination of wounds with their own microbiota. In an attempt to reduce wound infection, Joseph Lister introduced the practice of spraying surgical wounds with phenol solution. This used to make the hands of the surgeons very sore, and so they adopted the habit of wearing rubber gloves to protect themselves. The fact that the practice also reduces the risk of infection in a patient was a bonus.

It is not unusual for seriously ill patients to require artificial ventilation to assist with breathing. Hospital patients are also much more likely to have urinary catheters and intravenous devices inserted than people in the general population. All of these breaches increase significantly the risk that the patient affected will acquire an infection. Patients hospitalised because of serious burns are also vulnerable to colonisation and infection with bacteria from the hospital environment.

Specialised nosocomial bacteria often cause these infections. These bacteria are rarely found to cause infection in the population at large but are often seen causing infections in hospitalised patients. Frequently they are adapted to the hospital environment and are often resistant to a range of antibiotics. The pattern of antimicrobial susceptibility can be difficult to predict in nosocomial pathogens. This can make treatment of nosocomial infections very difficult. In the case of vancomycin-resistant enterococci, a newly emerged pathogen on high-dependency units, there are no conventional antimicrobial therapies that can be used to control the infections it causes.

Important nosocomial pathogens include pseudomonads and members of the Enterobacteriaceae including Klebsiella spp., Citrobacter spp., Enterobacter spp., and Serratia spp. as well as the infamous meticillin-resistant Staphylococcus aureus.

Illness that requires hospitalisation may cause suppression of our innate and acquired defences, increasing the chances that hospitalised patients develop infection. This may be because of the nature of the disease or because of the properties of therapeutic agents, such as the immunosuppressive drugs given to transplant patients, for example. Even the act of lying down compromises the function of the mucociliary escalator, leading to an increased risk of developing a chest infection. Likewise, the urinary bladder relies upon gravity to empty properly. If micturition takes place in a prone position, then the bladder does not empty properly and the patient is prone to urinary tract infection.

Sterilisation of surgical instruments and other medical items ensures that pathogens are not accidentally introduced into the patient undergoing medical procedures. The use of contaminated instruments have been implicated in a number of infection incidents. Endoscopes that have not been properly sterilised have been responsible for the spread of salmonella infections, through contamination with faecal material from a carrier inoculating subsequent patients submitting to endoscopy. Neurological instruments have been responsible for the spread of Creutzfeldt-Jakob disease, the human form of mad cow disease. Similarly, pituitary hormone preparations, when collected from a large pool of cadavers have been responsible for this relentless and ultimately fatal neurological disease.

Often simple measures can be employed to prevent cross-infection from one patient to another. Surgical gloves prevent microbes from the surgeon's hands from entering wounds. The single most important measure in the control of infection, however, is adequate hand washing. This has been known about for many years.

In the 1840's Ignaz Semmelweiss noticed that rich Viennese women were much more prone to infection following childbirth than the poor women in labour in the same hospital. The only difference that he noticed was that the poor were denied the 'privilege' of being treated by medical students. These students took a keen interest in anatomy and Semmelweiss also noticed that they did not wash their hands after dissection and before attending to a woman in labour. He instituted a hand-washing protocol and the incidence of puerperal fever dropped dramatically.

The control of nosocomial infection is a full time job. Each hospital has an infection control team. This may seem to be an extravagance but when one considers the number of patients affected by nosocomial infection and remembers the cost of keeping someone in hospital then the cross-infection control team can be seen as an . It is the role of the hospital infection control team to monitor the level of nosocomial infection and, when incidents arise, to respond by identifying the source of the infection and its mode of spread. With this information, it is usually a simple matter to break the cycle of infection.

Patients who are suffering from highly contagious infections are often barrier nursed. Barrier nursing techniques have evolved to protect the environment and subsequently the hospital personnel that come into contact with the patient from contamination or infection with a dangerous pathogen. Anything that comes into contact with the patient is considered contaminated and is sterilised before being returned into general use. Special clothing, gloves and masks protect personnel from nosocomial infection.

The lessons learned from barrier nursing techniques can also be used, in reverse barrier nursing, to protect patients who are most vulnerable to infection, such as organ-transplant recipients immediately following the organ graft. This protects the patient from the environmental dangers. It cannot be stressed strongly enough, however, that the single most important measure in the control of infection, however, is adequate hand washing.

There are a number of infections where the personnel of a hospital are at greater risk than are the general public. Important amongst these infections are hepatitis and infection with the human immunodeficiency virus, ultimately responsible for AIDS. Hospital personnel are much more likely than the general public to come across infected material that acts as a vector for these infections. There is good news for the control of hepatitis B. There is currently an effective vaccine against this infection. Unprotected people who are infected with the hepatitis B virus are at a vastly increased risk of developing carcinoma of the liver in later life. For infections caused by hepatitis C virus and for HIV infection the only protection currently available is constant care and vigilance.

Not all the viruses that cause hepatitis need concern hospital staff. Although the target organ is always the liver in hepatitis, different hepatitis viruses have different modes of transmission. Hepatitis B virus, a DNA virus, is spread via body fluids and blood, as is the RNA virus that causes hepatitis C. Both constitute a risk to hospital personnel, although there is an effective sub-unit vaccine that protects against hepatitis B. In contrast hepatitis A virus, a small RNA virus, spreads by the faecal-oral route and is of no more threat to hospital staff than to anyone else.

Although infections caused by meticillin-resistant Staphylococcus aureus and vancomycin resistant enterococci can have devastating effects on patients, they typically do not affect healthy adults. Healthy individuals have competent host defences and these are compromised by disease or medical or surgical procedures. Hence, sick people may suffer life-threatening infections while hospital staff may be unaffected by these opportunistic pathogens.

One of the greatest triumphs of modern medicine has been the introduction of a rational system of antimicrobial chemotherapy to combat infectious diseases. Since time immemorial, folk remedies have exploited moulds or mould extracts to treat infections. In the early days of microbiology, attempts were made to use extracts derived from fungal cultures to prevent surgical wound infection. Joseph Lister used cultures of his own urine to investigate the microbiology of air. He noted that if moulds were present in his cultures, the bacteria that were also there appeared non-motile and degenerate, whereas if bacteria grew without moulds, they were highly motile. Lister concluded that moulds produced a substance or substances that adversely affected the viability of bacteria. He then reasoned that culture filtrates obtained from moulds should prevent infection if used to irrigate surgical wounds. This practice started sixty years before Alexander Fleming described the antibacterial properties of penicillin, produced from a mould that he had originally misidentified.

The problem of producing sufficient antibiotic from mould cultures defeated both Lister and Fleming. Indeed, Fleming was slow to appreciate the clinical applications of his observations. He intended that penicillin should be used as a selective agent in laboratory media rather than to be administered to patients directly.

It was not until the early days of the Second World War that an allied Anglo-American effort overcame the problem of large-scale production and penicillin therapy for human infection was introduced. By the end of the War, penicillin was so plentiful that it was being used to cure cases of gonorrhoea in the allied troops. This was so that they could more quickly be returned to the front line than would otherwise be the case.

After the War, penicillin became generally available. This was not so in the first person to be treated: a policeman with overwhelming staphylococcal sepsis. The antibiotic was in such short supply that it had to be re-purified from the patient's urine. Although in this instance the antibiotic caused relief of the clinical condition, once the supply of penicillin had run out, the staphylococcal infection regained its hold and the patient died.

Penicillin represented the first true antibiotic: a substance produced by one microorganism that, in very small amounts, inhibits or kills other microorganisms. Those agents that kill bacteria are said to be bactericidal and those whose effects are reversible upon removal of the drug are bacteriostatic. The graphs below show the growth curves for a bacterium treated with two drugs. The upper curve shows the activity of a bacteriostatic drug. The bacterial growth resumes when the drug is withdrawn. The cidal drug, shown in the lower graph, kills bacteria from the time of administration to the culture.

One measure of the efficacy of an antibiotic is to determine the MIC, or Minimum Inhibitory Concentration of the drug. This is the concentration at which growth is prevented. Another measure is the Minimum Bactericidal Concentration, or MBC. This is defined as the minimum concentration of a drug required to kill all the cells in a culture. If an antibiotic is bactericidal, then the MIC and MBC values will be very close. If a drug is bacteriostatic, however, then it will inhibit growth without killing cells in a culture. With such cases, the MIC will be significantly lower than the MBC.

A number of very important antibiotics used in clinical practice are bacteriostatic. They are generally effective because their use 'buys time' while our own defences eliminate the infection. An important exception is in bacterial endocarditis. In this condition, the infecting bacteria become protected by a layer of fibrin than deposits at the site of infection. This protects the pathogen from the host defences. If endocarditis is to be treated successfully, it is essential to use a bactericidal regime and have a prolonged course of antimicrobial therapy.

The development of antibiotics was carried out in parallel with the search for chemical antibacterial agents: artificial compounds that inhibit or kill microbes. Paul Ehrlich described such compounds as magic bullets. The most successful of the early antimicrobial compounds, the sulphonamides, are still in use today. Often the term antibiotic is applied very loosely and includes antibacterial agents as well, although this is strictly incorrect.

Bacteria are good targets for the activity of antimicrobial substances. Aspects of their metabolism are significantly different from that of humans. Antibiotics may act upon bacterial reactions that are not found in human cells. This provides the basis for the selective toxicity of antibiotics, affecting the bacteria but not the human host.

Not all antibiotics are without their side effects. For example, penicillin allergy is very common in humans. The adverse effects of antibiotics are not necessarily associated with their antimicrobial properties. Penicillin allergy is due to the presence of the thiazolidine ring of penicillins. It is the b-lactam ring rather than the thiazolidine ring that is responsible for the antibiotic activity.

Fungi and protozoa have a metabolism that is much closer to that of humans than do bacteria, Moreover, viruses are obligate intracellular parasites that depend almost exclusively upon human metabolism for their replication. Consequently, anti-virus, antifungal and anti-protozoal drugs are more limited in their scope and are generally more toxic to humans than are antibacterial drugs.

In order to protect ourselves and others from infectious agents, a range of sterilisation and disinfection procedures have been devised. Sterilisation is the complete removal of all living cells and viruses from an object or environment. This also refers to the inactivation of the agents that cause spongiform encephalopathies. Disinfection is a less precise term referring to the removal or inhibition of organisms likely to cause disease. Antisepsis is disinfection of living tissue. In general, antiseptics are less harsh in their activity than disinfectants. Sanitisation is a term used particularly in the food industry. This refers to the inhibition or removal of microbes likely to harm the public health: another vague term. Sterilisation and disinfection may be achieved using physical or chemical methods. Chemicals used for disinfection and sterilisation are used at greater concentration that are antibiotics and they typically lack the selective toxicity of antibiotics as well.

Fire has been used since time immemorial to purge items. Cooking meat reduces the risk of gastrointestinal infection, for example and travellers to third world countries are advised to boil drinking water. Heat is commonly used to sterilise materials. It causes disruption of membranes, coagulation of proteins and disruption of nucleic acids. Moist heat is more effective at sterilising than dry heat. Autoclaves work on the same principle as the domestic pressure cooker. Steam under pressure is used to sterilise items. A temperature of 121 degrees Celsius (15 lb/sq in pressure) for fifteen minutes is generally adequate to kill the most resistant bacterial spores. The agents of spongiform encephalopathies require 135 degrees for 1 hour! Hot air ovens are used to heat-sterilise items that are damaged by steam. A typical sterilisation regime is 160 degrees Celsius for two hours.

Sterilisation temperatures above the boiling point of water are required to inactivate bacterial spores. Because of this you should note that boiling drinking water does not sterilise it. It simply kills all the microbes likely to cause water-borne infections.

• Ultra-violet irradiation causes damage to DNA leading to the formation of pyrimidine dimers. This leads to genetic damage and ultimately to cell death. A wavelength of 260 nm is used to sterilise air and work surfaces. Ultraviolet light has very poor penetrative properties.
  
  
• X-rays are more effective than ultraviolet light at causing the death of microbes, but are costly to produce and difficult to control. They have little place in routine sterilisation.
  
  
• Gamma irradiation techniques are sometimes referred to as cold sterilisation. Radioactive cobalt is a by-product of the nuclear industry and is a cheap source of gamma rays. They have the advantage that they penetrate materials well. Gamma irradiation is used in the sterilisation of many medical equipment items.

Bacteria and larger microbes can be removed from solutions by passing the liquid trough appropriate filters. Nitrocellulose is now commonly used. With modern manufacturing techniques the pore size can be carefully controlled, and filters of different capacity can be obtained. Filtration does not remove viruses. The word 'virus' was once used to mean a poison. What are now called viruses were originally termed 'filterable viruses' because their ability to cause disease was not prevented by passage through a filter that would trap bacteria.

Chemical disinfection began with Joseph Lister's experiments to reduce wound infections following surgery. He sprayed a dilute phenol solution over the operation sites, with remarkable results. On of the side effects, however, was that the surgeons' hands suffered phenol burns. It was for this reason that surgical gloves were first introduced. Their added benefit in preventing wound infections in the surgical patient has outlived their role in protecting the surgeon.

Phenol disrupts cells and denatures proteins destroying enzyme activity. It has mild anaesthetic properties but it is toxic to humans and is highly corrosive. Because it was the first disinfectant and also because of its antimicrobial properties, phenol remains the gold standard against which other disinfectants are measured. Derivatives such as chlorhexidine and hexachlorophene lack the toxicity of the parent compound. Chlorhexidine can be used in aqueous solution or in iso-propanol Phenol is active in killing bacterial spores.

Methanol, ethanol and iso-propanol are efficient killers of microbes. They act by dehydrating cells, disrupting membranes and coagulating proteins. They act more efficiently as a 70% (v/v) aqueous solution than as absolute alcohols. Bacterial spores and many viruses are, however, resistant to the killing effects of alcohol. Spores have very little water and hence are not killed by the dehydration caused by alcohol treatment.

• Chlorine and iodine are powerful oxidising agents. It is in this capacity that they kill microbes. They can rapidly be inactivated by the presence of organic matter, and can be highly irritant to humans. Chlorine is added to municipal tap water, and at higher concentration is found in swimming pools. The most common chlorine disinfectant is sodium hypochlorite - household bleach. This is used to disinfect spillages of body fluids in the hospital environment, and 'kills all known germs - dead'!

• Tincture of iodine is an old-fashioned antiseptic containing 2.5% iodine and 2.5% potassium iodide in 90% ethanol. The highly irritant nature of iodine preparations can be eased using an iodophore. Iodine is trapped within a polymer that slowly releases the halogen. This reduces the irritation and increases the time over which iodine is active.

The aldehydes formaldehyde and glutaraldehyde are two powerful disinfectants. They cause denaturation of nucleic acids and proteins. Both are used in solution, but formaldehyde vapour can be used in a 'bomb' to decontaminate rooms, for example.

Quaternary ammonium compounds such as cetrimide and benzalkonium chloride are organically substituted ammonium compounds. They act as cationic detergents, and are popular as home disinfectants because they have pleasant smells, but bacteria of the genus Pseudomonas can utilise these compounds as a carbon nitrogen and energy source.

There are very few gasses that are toxic to microbes. Ethylene oxide is an exception. It is highly flammable and in practice it is mixed with 90% carbon dioxide. It is used to sterilise bulky items or delicate instruments. It is expensive and takes a considerable time to desorb from surfaces. With some complex instruments, penetration of gas into channels is ineffective, particularly if the channels are wet or have extraneous matter associated with them.

The important material for medical physicists to know are summarised below:

• How are microorganisms different from each other and how may they be distinguished from higher organisms?
  
  
• What makes a virus a virus and not a bacterium?
  
  
• How do fungi differ from bacteria and viruses?
  
  
• What are the essential cellular features of bacteria?
  
  
• How do infections spread?
  
  
• How are micorbes controlled?
  

You should remember particularly person-to-person spread through airborne droplets, the faecal-oral route, sexual transmission, direct inoculation and the transplacental route. You should also remember that food and water may be the vectors of infection, as may insects.

You should be aware that hospitalised people are more vulnerable to infection than others but that hospital personnel are exposed to particular infections because of the nature of their work. These would include, for example HIV infection and infections caused by hepatitis B and hepatitis C viruses. It would not, however, include hepatitis A. This is because the former viruses are spread via body fluids whereas hepatitis A is spread via the faecal-oral route.

You should understand the difference between an antibiotic and a disinfectant. Antibiotics work at much lower concentrations that disinfectants and they tend to have very much more precise targets, disrupting particular cellular functions. Disinfectants, in contrast, act by denaturing cellular components. You should remember that some antibiotics kill bacteria, the bactericidal agents, whereas others, the bacteriostats, merely inhibit growth. Comparing the MIC and MBC of the drug in question can differentiate these. If it is bacteriostatic, then the MIC will be significantly lower than the MBC. If, however, the antibiotic is bactericidal, then the MIC and MBC values will be the same or very similar.

You should also have a broad grasp of the range of physical and chemical methods of destroying pathogens. Moist heat is more effective than dry heat; ultra-violet irradiation is more easily controlled than x-rays. Gamma irradiation has an important role in sterilising hospital equipment. Phenol is the 'gold standard' for chemical disinfection but it is highly toxic. Disinfection of delicate instruments may prove particularly difficult.