Impedance microbiology is a rapid method that enables qualitative and quantitative tracing of microorganisms by measuring the change in the electrical conductivity. With direct impedance technology, the change in the conductivity of a liquid culture medium serves as a measuring parameter, whereas with indirect impediometry, the change in the electrical conductivity of a reaction solution, which occurs through the absorption of gases from the inoculated bacterial culture, is measured. Most investigations concerning the applicability of impediometry in food microbiology deal with the impedimetric detection or enumeration of Enterobacteriaceae, especially the detection of Salmonella. However, impediometry has been applied to other bacterial groups or species as well. Furthermore, a great number of published findings concern the impedimetric determination of the total bacterial count. The successful application of this fast method on further areas of food hygiene, such as tracing antibiotics and testing additives for their antimicrobiological effect, has also been described. In general the use of impediometry for the application areas stated has been judged positively. However, the time and expense required by the user to optimize the method, the deficits when testing slightly contaminated sample material or determining the bacterial count in those cases in which the microorganisms are sublethally damaged, and the necessity of performing individual calibration for each food category limit the applicability of impediometry.

The prevalence of the enterotoxin gene in a well-characterized collection of 71 Clostridium perfringens strains from 36 separate food-poisoning cases or outbreaks was analyzed with the polymerase chain reaction (PCR). The clonality of 39 strains originating from 14 outbreaks where at least two isolates were available was studied with pulsed-field gel electrophoresis (PFGE) using SmaI and ApaI restriction endonucleases. The cpe gene PCR assay was found to correlate well with Clostridium perfringens enterotoxin (CPE) production in vitro with reverse passive latex agglutination. Of the C. perfringens food and clinical food-poisoning isolates 24 (86%) and 38 (88%) were cpe-positive, respectively. Different PFGE patterns indicated that multiple cpe-positive clones are frequently present within one outbreak. The existence of cpe-positive and negative isolates with identical or nearly identical PFGE patterns in a single outbreak suggests that the cpe gene may be in a movable genetic element.

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Plasmid profiling was used for the characterization of Clostridium perfringens isolates involved in disease outbreaks. The usefulness of this technique was demonstrated by the retrospective examination of food and patient isolates from 10 cases and outbreaks from 1984 to 1991. The origin of three outbreaks could be clearly confirmed due to identical plasmid profiles in all isolates. In one outbreak identical plasmid patterns were found between one food and one patient isolates, while one plasmid was missing in the second patient isolate. In an additional two cases a relationship between food and patient isolates is likely, if the possibility of the loss of one plasmid in one of the isolated strains is considered. In one outbreak two faecal isolates could be related to an isolate from one of the two foods implicated as outbreak source; isolates from the other food and a third faecal sample could not be linked to any other isolate. The results from three outbreaks were largely inconclusive because plasmids were not present either in all or in some of the isolates.

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