Cross-infection in health care practice

Nurse E was a student attached to the Surgical Unit of a District General Hospital. Shortly after she was posted to the Unit, an increased number of patients were found to be suffering from post-operative wound infections. Analysis of the culture reports indicated that most cases were caused by meticillin-resistant Staphylococcus aureus.

Because of the increased incidence of wound infections, and because they were caused by MRSA's, the hospital Infection Control Team initiated an investigation. Sub-typing showed that all of the MRSA's were clonal, that is they all belonged to the same strain, as far as could be determined with the techniques available. Despite initiating a surveillance programme on the Unit, at first no source could be found for the MRSA's .

A monitoring programme to check the efficacy of hand-washing was then initiated. After each time that they washed their hands, staff on the Unit were requested to make fingertip impressions on mannitol salt agar, a medium selective for staphylococci. No MRSAs were isolated for five days, during which MRSA wound infections continued on the Unit. Then, when collecting the impression plates one afternoon, one of the Hospital Infection Control Team observed that after she had made her fingertip impression, Nurse E rubbed her hands with a moisturising cream, she said it was "...because of the roughening of my skin". She suffered from intermittent bouts of eczema, and had just recovered from an episode before starting on the surgical Unit.

The Infection Control Officer asked if she could sample the hand cream, and it yielded a culture of MRSA, indistinguishable from the clone that was isolated from patients on the Unit. Use of the moisturising cream on the Unit was banned, and all the staff were required to wash their hands using alcohol-based chlorhexidine. Monitoring of handwashing with fingertip impression plates continued for a week, but following the introduction of control measures, no further cases of MRSA wound infection were seen on the Unit.

This incident occurred on a Special Care Baby Unit, involving six isolates of Proteus mirabilis. These have been typed using the Dienes method. This exploits the swarming phenomenon that proteeae display. Strains of the same Dienes type swarm over each other, whereas strains of different Dienes types have clearly delineated borders when grown on fresh blood agar. In this way, strains of proteeae may be distinguished.

To determine the relationship of the six isolates provided two fresh blood agar plates have been inoculated with the six strains, using the following pattern of inoculation:

This ensures that each strain can interact with every other strain. When inoculating these plates, it is important that the inoculum is light, and that the area of inoculation should not exceed 2-3 mm. The plates are incubated at 30^oC overnight before being examined. The lower temperature of incubation than normal enhances the production of the "line of demarcation" between different strains.

Isolates of Proteus mirabilis were made from four babies nursed on the Special Care Baby Unit of a District General Hospital. The incident extended over a one-month period starting in early March. One of the babies died from septicaemia caused by Proteus mirabilis. Preliminary epidemiological investigation indicated that only two nurses were responsible for the care of all four babies from whom the Proteus mirabilis was isolated. Skin swabs were taken from the hands of both nurses, and both were required to submit faecal specimens for bacteriological investigation.

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The epidemiological information regarding these isolates is given below:

After incubation, the Dienes plates appeared as below.

Schematically, the swarming edges of the advancing colonies are shown as fainter lines; ditches separating colonies of different Dienes types are indicated by bolder lines: