Medical Microbiology - A Brief Introduction

Bacteria are generally simple structures. The bacterial cell lacks a membrane-bound nucleus. Because of this, bacteria are described as prokaryotes. Despite their simplicity, bacteria have an enormous range of metabolic capacities, and can be found in some of the most extreme environments on earth. Only a small minority of bacteria causes disease.

There are three basic shapes that bacterial cells adopt. They are either round, rod shaped or spiral. Round bacteria are referred to as cocci (singular: coccus), and rod shaped bacteria are known as bacilli (singular: bacillus). The term 'bacillus' meaning a rod-shaped bacterium should NOT be confused with the genus of bacteria known as 'Bacillus'. The shape of bacterial cells is of fundamental importance in the classification and identification of bacteria. Although bacteria are of three basic shapes, they display an astonishing variety of forms when viewed microscopically.

Diplococcal cells of
Streptococcus pnuemoniae

Streptococci

The vast majority of bacteria have a cell wall containing a special polymer called peptidoglycan. The cell wall lies outside the cell membrane, and the rigid peptidoglycan is important in defining the shape of the cell, and giving the cell mechanical strength.

The bacterial cell wall is a unique biopolymer in that it contains both D- and L-amino acids. Its basic structure is a carbohydrate backbone of alternating units of N-acetyl glucosamine and N-acetyl muramic acid. The NAM residues are cross-linked with oligopeptides. The terminal peptide is D-alanine although other amino acids are present as D- isomers. This is the only biological molecule that contains D-amino acids and it is the target of numerous antibacterial antibiotics. The cell wall of Gram-positive bacteria lies beyond the cell membrane and is largely made up of pepidoglycan. There may be up to 40 layers of this polymer, conferring enormous mechanical strength on the cell wall. Other polymers including teichoic and teichuronic acids also lie in the cell walls of Gram-positive bacteria. These act as surface antigens.

Cells with many layers of peptidoglycan can retain a crystal violet-iodine complex when treated with acetone. These are called Gram-positive bacteria and appear blue-black or purple when stained using Gram's method. Gram-negative bacteria have only one or two layers of peptidoglycan and cannot retain the crystal violet-iodine complex. These need counterstaining with another dye to be seen using Gram's method. A red dye such as dilute carbol fuchsin is often used.

The cell wall of Gram-positive bacteria lies beyond the cell membrane and is largely made up of pepidoglycan. There may be up to 40 layers of this polymer, conferring enormous mechanical strength on the cell wall. Other polymers including teichoic and teichuronic acids also lie in the cell walls of Gram-positive bacteria. These act as surface antigens.

In contrast to Gram-positive cells, the cell envelope of Gram-negative bacteria is complex. Above the cell membrane is a periplasm. This area is full of proteins including enzymes. One or two layers of peptidoglycan lie beyond the periplasm. Gram-negative bacteria are thus mechanically much weaker than Gram-positive cells. Beyond the peptidoglycan of the Gram-negative cell wall lies an outer membrane. This has protein channels - porins - through which some molecules may pass easily. The outer side of the Gram-negative outer membrane contains lipopolysaccharide. This provides the antigenic structure of the surface of Gram-negative bacteria and also acts as endotoxin. It is this that is responsible for eliciting the symptoms of Gram-negative shock if it gains access to the bloodstream. Porins and Outer Membrane Proteins (OMPs) act as transporters through the outer membrane.

The cell envelope of a Gram-negative bacterium

The nature of the cell wall is also reflected in different cell architectures.

A Gram-positive cell

A Gram-negative cell

A few medically important bacteria do not stain easily using conventional stains, and need to be heated to near boiling point in the chosen dye (carbol fuchsin for light microscopy: rhodamine-auramine for fluorescence microscopy) for at least five minutes. This is to allow the dye to penetrate the waxy cell walls. Having taken the stain, these bacteria resist decolourisation with both acids and alcohol, and are known as acid-alcohol fast bacteria. This is a property of mycobacteria. These include Mycobacterium tuberculosis, the cause of tuberculosis; a chronic infection. Most common is pulmonary tuberculosis, affecting the lung. The kidneys may be infected in renal TB, and there is a rare form of osteomyelitis (bone infection) and meningitis caused by TB. In miliary tuberculosis, the infection is disseminated through the body. Another medically important mycobacterium is Mycobacterium leprae, the cause of leprosy; a chronic infection of the skin and nerves. Nerve damage leads to a loss of sensation, and ultimately to paralysis. This can lead to tissue damage that can lead to the loss of fingers and toes.

The bacterial chromosomal DNA is located in a region of the cell known as the nucleoid. Bacteria, being prokaryotes, do not have a true, membrane-bound nucleus. Bacteria carry a single chromosome that is circular in structure.

The cytoplasm of bacteria contains polysomes - a range of ribosomes actively translating messenger RNA into proteins. Some bacteria also have inclusion bodies within the cytoplasm. These are often energy storage resources. Some inclusion bodies are referred to as metachromatic granules since they change the colour of dyes used to stain cells. Inclusion bodies found within Corynebacterium diphtheriae, the cause of diphtheria, are an important example of metachromatic granules.

Flagella are responsible for the motility of pathogenic bacteria and can play a role in the production of disease. Gram-negative pathogenic bacteria may be covered in fine hairs called fimbriae (singular: fimbria) these help to stick to body surfaces. Pili can attach two bacterial cells together: sex pili are necessary for the transfer of certain plasmids between bacteria.

Bacterial cells may carry a single flagellum, and are thus described as monotrichous. If the single flagellum is at one end of a rod-shaped cell it is known as a polar flagellum. If the bacterium carries a single tuft of flagella, it is said to be lophotrichous (lophos - Greek for a crest). When the tuft appears at both ends of the cell, the bacterium is amphitrichous (amphi - Greek for 'at each end'). Bacteria that are covered all over in flagella are said to be peritrichous (peri - around).

Some bacteria are enclosed within a capsule. This protects the bacterium, even within phagocytes, helping to prevent the cell from being killed. Encapsulated bacteria grow as 'smooth' colonies, whereas colonies of bacteria that have lost their capsules appear rough. Rough colonies do not generally cause disease. Encapsulated bacteria do not succumb to intracellular killing as easily as bacteria that lack capsules. Strains of Streptococcus pnuemoniae that lack capsules do not cause disease. All the bacteria that cause meningitis are encapsulated. Suspending bacteria in India ink is an easy way of demonstrating capsules. Ink particles cannot penetrate the capsular material and encapsulated cells appear to have a halo around them. This is the Quellung reaction.

A few species of bacteria have the ability to produce highly resistant structures known as endospores (or simply spores). These resist a range of hazardous environments, and protect against heat, radiation, and desiccation. Endospores form within (hence endo-) special vegetative cells known as sporangia (singular sporangium). Diseases caused by sporing bacteria include botulism (Clostridium botulinum), gas gangrene (Clostridium perfringens), tetanus (Clostridium tetani) and acute food poisoning (Clostridium perfringens, again) All these bacteria are 'anaerobic'.

Some bacteria have an absolute requirement for oxygen. These are the obligate aerobes. Others, the facultative anaerobes can survive in the absence as well as the presence of oxygen. The obligate anaerobes are killed by traces of oxygen. A small group of bacteria are killed by normal atmospheric levels of oxygen, but yet require traces of oxygen to grow. These are the referred to as microaerophiles.