Faster food-borne bacteria identification

A technique has been developed that could help scientists quickly diagnose the bacteria believed to be the most common cause of foodborne gastroenteritis in westernised countries.

Erin Price, from Queensland University of Technology's Faculty of Science, has developed a method that uses genotyping to identify the bacteria Campylobacter jejuni.

"Campylobacter jejuni is the commonest cause of bacterial foodborne gastroenteritis in westernised countries," said Price.

"It is more common than salmonella, yet most people have never heard of it."

Price said there were about 20,000 registered cases of campylerbacteriosis in Australia each year, but scientists thought the real rate could be 10 times higher.

"The reason the rate is so low is that it rarely kills people and is self-resolving, so in many cases it is not diagnosed," she said.

"Symptoms include diarrhoea, which can last anywhere from two days to two weeks."

Despite the prevalence of the bacteria, scientists are still unclear as to how C. jejuni propagates, or how it is transmitted to humans, or if some strains are more pathogenic than others.

Price's thesis points out that campylobacters are unlikely foodborne pathogens since they seem to possess fastidious growth requirements, are sensitive to environmental stresses such as desiccation, osmotic stress, and even oxygen, and as yet do not appear to harbour strain-specific virulence or pathogenic determinants.

"We think that it is mainly transmitted in foodstuffs, and predominantly from improperly handled and undercooked poultry, although there are potentially many other sources of infection," Price said.

One factor stopping scientists from discovering more about C. jejuni is the lack of a standardised, simple and cheap method of fingerprinting the bacteria.

The current method of choice for outbreak investigations of most bacteria, according to Price, is the PFGE (pulsed field gel electrophoresis) procedure due to the high resolution obtained when used for strain characterisation.

"The value of PFGE for routine characterisation of campylobacteriosis, however, is questionable as this technique is sensitive to subtle genetic changes, potentially obscuring existing strain relationships and hampering epidemiological studies of C. jejuni populations over time," she said.

"PFGE relies on the comparison of electrophoresis banding patterns and therefore the isolate profiles are subject to potential ambiguities in interpretation, particularly when comparing results between laboratories."

Price developed a highly reproducible method for characterising C. jejuni using high-resolution melt (HRM) analysis, which provided stain discrimination that was comparable to the PFGE procedure.

The procedure suggests that the relatively new HRM technology could be a cheaper, more efficient alternative to DNA sequencing.

"The approaches are not restricted to particular organisms and can potentially be applied to any species for which comparative genetic data is available," she said.

Price also developed an SNP (single nucleotide polymorphism) real-time PCR-based assay as a faster and cheaper alternative to the MLST (multilocus sequence typing) genotyping method, by identifying seven SNPs from the C. jejuni/C. coli database that provided a Simpson's Index (D) of 0.98 when compared to a complete MLST characterisation.

The research also found eight binary genes characterised by CGH (comparative genome hybridisation) that provided a D of 1 compared to complete CGH characterisation.

"What I have done is develop a systematic and novel genotyping method to be used by clinicians and scientists, which essentially creates a fingerprint of the bacteria using genetic markers," said Price.

"This fingerprint can then be used to characterise the bacterial strain present."

Price hopes her methods would be used to effectively determine the origin of what seem to be sporadic cases of C. jejuni, ultimately determining the route of transmission to humans.

"If we can figure out where the bacteria are coming from we can look for ways of preventing its spread," she said.