Growth of pure cultures of Verocytotoxin-producing Escherichia coli in a range of enrichment media.

AIMS: This study compared the growth of different strains of Verocytotoxin-producing Escherichia coli (VTEC) in a range of selective enrichment media. METHODS AND RESULTS: Turbidometric and impedance methods were used to determine the growth of VTEC in pure culture in different enrichment media. Ten strains failed to grow in buffered peptone water + vancomycin, cefsulodin, cefixime at 42 degrees C and some failed to grow, or grew poorly in E. coli (EC) medium supplemented with 20 mg l(-1) novobiocin and modified EC supplemented with 20 mg l(-1) novobiocin at 37 degrees C and 42 degrees C. Individual VTEC strains were sensitive to the selective agents in some media. Statistical analysis of the conductance detection times of 10 strains showed no overall effect of temperature alone (P = 0.66) but there were significant (P < 0.001) effects as a result of the combination of medium and temperature and these two factors were influenced by strain. CONCLUSIONS: Growth of VTEC during enrichment is dependent on different factors alone or in combination. These include medium type, presence of certain selective agents or antibiotics, incubation temperature and the initial population of VTEC. Sensitivity to these conditions can be strain related. SIGNIFICANCE AND IMPACT OF THE STUDY: This study highlighted differences in the ability of some enrichment media to support the growth of VTEC, making them unsuitable for the isolation of VTEC, especially low numbers of non-O157 strains.

Survival of Escherichia coli O157:H7, O111:H- and O26:H11 in artificially contaminated chocolate and confectionery products.

To date, the survival of Escherichia coli O157:H7 and other verocytotoxin-producing E. coli (VTEC) in chocolate and other confectionery products has not been fully established, unlike Salmonella, which have been responsible for occasional outbreaks of infection linked to contaminated chocolate and related products, although none of these outbreaks have been related to products produced in the United Kingdom. The United Kingdom Biscuit, Cake, Chocolate and Confectionery Alliance commissioned this study to obtain information on the decline and potential survival of E. coli, particularly verocytotoxin-producing strains, in reduced aw confectionery products chocolate, biscuit cream and mallow. These products were artificially contaminated with high (4 log10 cfu/g) and low (2 log10 cfu/g) levels of E. coli O157:H7, O111:H- and O26:H11 and their survival, as affected by storage temperature (10, 22 and 38 degrees C), was monitored over 12 months. Preliminary studies to establish the best inoculation and recovery procedures indicated that differences between counts on selective and non-selective media used were not sufficiently different to influence the outcome of this study. Irrespective of sample type, rapid decline was observed in products stored at 38 degrees C and increased survival occurred in products stored at 10 degrees C. In chocolate (average aw 0.40), these bacteria were detected for up to 43 days in samples stored at 38 degrees C. At 22 degrees C they survived for up to 90 days and in product stored at 10 degrees C they could still be detected after 366 days storage. In biscuit cream (average aw 0.75) they survived for 2 days at 38 degrees C, 42 days at 22 degrees C and 58 days at 10 degrees C. Whilst mallow (aw ca. 0.73) was not stored at 38 degrees C, these bacteria could still be detected in samples stored for up to 113 and 273 days at 22 and 10 degrees C, respectively. The observed prolonged survival of these bacteria under conditions of reduced aw and lowered storage temperature in this study is supported by previous studies with Salmonella and E. coli O157:H7 in other foods. In the same way that Salmonella bacteria can survive for long periods, in excess of 12 months, in chocolate, this study provides evidence that E. coli, including pathogenic strains, can also survive for similar periods of time. Assuming the routes of transmission are similar, controls currently used by the confectionery industry to prevent contamination by Salmonella should also be effective against E. coli, including VT-producing strains, providing that all raw materials have been suitably processed, stored and handled before and during manufacture.

Comparison of methods for the recovery and detection of low levels of injured Salmonella in ice cream and milk powder.

This study compared the ability of four rapid methods and a standard cultural method to detect low levels of heat-injured cells of Salmonella typhimurium in ice cream and skimmed milk powder. The detection of Salmonella in samples contaminated with low levels (< 10 cfu 25 g-1) was significantly greater with the novel broth method than with the other methods (P 10 cfu 25 g-1, there was no significant difference between the methods except for the novel broth method and a dipstick-based immunoassay (P

Evaluation of Petrifilm methods for enumeration of aerobic flora and coliforms in a wide range of foods.

Petrifilm is a ready-to-use alternative to traditional microbial enumeration methods. The Petrifilm Aerobic Count Plate (ACP) and Coliform Count Plate (CCP) were compared with standard methods for the enumeration of the aerobic mesophilic flora and coliform bacteria in 91 foods covering a wide range of different food commodities. There was good correlation between the Petrifilm ACP and the standard aerobic colony count method (r = 0.989) and between the Petrifilm CCP and the standard Violet Red Bile Agar plating method (r = 0.872). In both cases, the Petrifilm methods had a better repeatability than the standard methods. The Petrifilm ACP and CCP were shown to be practical and accurate alternatives to standard enumeration methods in a wide range of foods, with benefits of saving time, labour and incubator space.

Comparison of two commercial preparations of buffered peptone water for the recovery and growth of Salmonella bacteria from foods.

This study compared the performance of two commercial preparations of buffered peptone water. Performance was assessed in terms of ability to resuscitate and recover low numbers of stressed cells, buffering capacity, growth of Salmonella bacteria in pure culture and growth of Salmonella in food pre-enrichments. Although both the preparations of BPW had similar chemical compositions, differences in their recovery performance were found. Brand A recovered significantly higher numbers of heat-injured Salmonella (mean = 0.57 log10 cfu ml(-1) difference) in pure culture compared with brand B when dealing with very low inoculum levels. Although brand B had higher buffering capacity, the pH at the end of the pre-enrichment was found to be similar in both media, even in foods such as milk powder which showed the greatest decline in pH. Both brands were comparable in their ability to grow unstressed Salmonella from different food types. In unstressed cell studies, similar cell numbers were recovered at the end of a 24 h incubation period from both media, although brand B yielded a higher biomass. In the food study with unstressed cells, performance was related more to the food type and the likely association between this and the level and type of competitor organisms present, rather than to the brand of medium used.

Comparison of three enrichment media for the isolation of Campylobacter spp. from foods.

AIM: This study compared the performance of three Campylobacter enrichment broths: Bolton broth (BB), Campylobacter Enrichment broth (CEB) and Preston broth (PB). METHODS AND RESULTS: Pure cultures of target and competitor organisms, and naturally-contaminated food samples, were used to establish the performance of these media. In pure culture the PB supported the growth of the greatest number of strains of Campylobacter spp. but failed to inhibit some competitor organisms. The CEB showed the opposite result, inhibiting all 15 competitor organisms used but failing to support the growth of five Campylobacter strains. By comparison, BB showed the best compromise between inhibition of competitors and growth of Campylobacter. CONCLUSIONS: Plates inoculated with BB and CEB food enrichments resulted in more Campylobacter growth than those inoculated with PB, which supported significantly less typical growth (P < or = 0.001). The most common competitor organism isolated from PB was Escherichia coli, and Pseudomonas spp. were frequently isolated from BB and CEB. Both BB and CEB were better than PB for the isolation of Campylobacter from naturally-contaminated foods, although BB yielded more confirmed Campylobacter growth than CEB. SIGNIFICANCE AND IMPACT OF THE STUDY: This study highlighted differences in performance of media used to isolate Campylobacter spp. from foods.