A medium for the isolation, enumeration and rapid presumptive identification of injured Clostridium perfringens and Bacillus cereus

Improved measurements of scant hydrogen peroxide enable experiments that define its threshold of toxicity for Escherichia coli

Escherichia coli is a model organism that has been exploited to reveal key details of hydrogen peroxide stress: the biomolecules that H₂O₂ most rapidly damages and the defensive tactics that organisms use to fend it off. Much less clear is the amount of exogenous H₂O₂ that is sufficient to injure the bacterium and/or to trigger its stress response. To fill this gap, we need to study the behavior of cells when they are exposed to defined amounts of H₂O₂ on an hours-long time scale. Such experiments are difficult because bacteria rapidly consume H₂O₂ that is added to test cultures. Further, lab media itself can generate H₂O₂, and media components interfere with the quantification of H₂O₂ levels. In this study we describe mechanisms by which media components interfere with H₂O₂ determinations, and we identify simple ways to minimize and correct for this interference. Using these techniques, it was shown that standard media generate so much H₂O₂ that most intracellular H₂O₂ derives from the medium rather than from endogenous metabolism. Indeed, bacteria spread on plates must induce their stress response or else perish. Finally, two straightforward methods were used to sustain low-micromolar steady-state concentrations of H₂O₂. In this way we determined that > 2 μM extracellular H₂O₂ is sufficient to trigger the intracellular OxyR stress response, and > 5 μM begins to impair cell growth in a minimal medium. These concentrations are orders of magnitude lower than the doses that have typically been used in lab experiments. The new approaches should enable workers to study how various organisms cope with natural levels of H₂O₂ stress.

Detection of Actinobacteria cultivated from environmental samples reveals bias in universal primers

AIMS

The aims of this study were to develop media to cultivate actinomycetes, screen the resulting isolates with Actinobacteria-specific primers, and examine the efficacy of detection of the actinobacterial isolates with universal primers.

METHODS AND RESULTS

Soil-extract medium was developed for a terrestrial bluff environment. Recovered isolates were subjected to polymerase chain reaction (PCR) with taxon-specific primers to identify Actinobacteria. Universal bacterial primers 24f and 1492r (modified and original versions) were used to amplify the 16S rRNA gene from the putative Actinobacteria. While both reverse primers failed to provide amplification products from 20% to 50% of the isolates, the 1492r primer detected Actinobacteria more effectively than 1492r-mod. The region of the gene containing the annealing site for the 1492r primers from 15 isolates that failed to amplify showed no differences in nucleotide sequence to the original 1492r primer.

CONCLUSIONS

Universal 16S rRNA gene primers are not capable of amplifying this gene from all bacteria within an environmental sample. Some Actinobacteria may share 100% sequence similarity to universal primers but remain undetected.

SIGNIFICANCE AND IMPACT OF THE STUDY

These findings are important for studies of particular taxa in environmental samples where reactions utilizing universal primers may not reveal the extent of their presence and diversity.

Killing of spores of Bacillus subtilis by peroxynitrite appears to be caused by membrane damage

During an infection of a higher eukaryote, dormant spores of a Bacillus species have been previously shown to be present in cells that can generate the toxic agent peroxynitrite (PON). Dormant spores of Bacillus subtilis were much more resistant to killing by PON than were growing cells, and spore-coat alteration or removal greatly decreased PON resistance. Spores were not killed by PON through DNA damage and lost no dipicolinic acid (DPA) during PON treatment. However, PON-killed spores lost DPA during subsequent heat treatments that caused much less DPA release from untreated spores. Although dead, the PON-killed spores germinated and initiated metabolism but never went through outgrowth; the great majority of germinated PON-killed spores also took up propidium iodide, indicating that they had suffered significant membrane damage and were dead. Together these data suggest that spore killing by PON is through some type of damage to the spore's inner membrane.

Occurrence of Clostridium perfringens in the broiler chicken processing plant as determined by recovery in iron milk medium

Over 30 years ago, Clostridium perfringens was reported as a contaminant of the processing plant and processed carcasses of broiler chickens. Poultry processing procedures and methods for detecting C. perfringens have changed since that time. Therefore, a study was conducted to determine the incidence and numbers of C. perfringens in the water of the scald tank, the water of the chill tank, and the rinse water of the processed carcasses from modern broiler chicken processing plants. In trial 1, collected samples were inoculated into iron milk medium (IMM) and incubated at 46 degrees C for 18 h (the traditional method) or at 37 degrees C for 3 h followed by incubation at 46 degrees C for 15 h (an injury recovery method). Each of three preselected broiler chicken flocks from two integrators were the first processed for that processing shift. The overall incidence of confirmed C. perfringens in samples associated with the three flocks was 40% of postprocessing scald water samples, 13% of preprocessing chill water samples, 13% of postprocessing chill water samples, and 19% of carcass rinses. The incidence of C. perfringens in samples incubated in IMM using the injury recovery procedure was significantly higher than in samples incubated in IMM by the traditional method, but only when all samples associated with the three flocks were pooled. In trial 2, water samples from each tank of a three-tank counterflow scalder, water samples from the prechill and chill tank, and samples of carcass rinses were collected in the middle of a processing shift during multiple visits to a processing plant. Samples were inoculated into IMM with neomycin and polymyxin B sulfate (IMMA) and incubated using the traditional and injury recovery procedures. The incidence of C. perfringens in water samples was 100% from scald tank 1, 100% from scald tank 2, 100% from scald tank 3, 88% from the prechill tank, and 63% from the chill tank. The incidence in carcass rinse samples was 67%. The mean most probably number (MPN) of C. perfringens for contaminated samples decreased from log10 5.07/100 ml of water in scald tank 1 to log10 1.26/100 ml of water in the chill tank. The mean MPN in carcass rinse samples was log10 1.20 C. perfringens per 100 ml. The incidence and mean MPN of C. perfringens in these samples after heat shock at 75 degrees C for 20 min was somewhat less, but high enough to indicate that much of the contamination arises from heat-resistant spores of this organism. In trial 2, there were no differences in incidence and MPN of C. perfringens in samples incubated in IMMA with the traditional method or the injury recovery method.

A rapid PCR-based DNA test for enterotoxic Bacillus cereus

The occurrence of DNA sequences encoding the hemolysin HblA complex and Bacillus cereus enterotoxin BceT, which have recently been confirmed as enterotoxins, was studied in Bacillus spp. To amplify these DNA sequences, PCR primer systems for the B component of hblA and for bceT DNA sequences were developed. The results from the amplification of hblA sequences correlated well with results obtained with the B. cereus enterotoxin (diarrheal type) test kit (RPLA kit), but not with the results of the Bacillus diarrheal enterotoxin visual immunoassay (BDE kit). Except for two thermophilic strains, all strains that were positive in PCR amplification assays with the hblA primers were also positive when tested with the RPLA kit. The hblA DNA sequence was found in 33 strains, and these strains were closely related according to 16S rDNA-RFLP analysis, except B. pasteurii. In PCR amplifications with the bceT primers only the model strain gave a positive signal. It is concluded that screening of the hemolysin HblA complex by the PCR method allows faster detection of enterotoxin production than does testing with the RPLA enterotoxin kit.

Recovery of thermally-stressed Escherichia coli O157:H7 by media supplemented with pyruvate

Unheated and heat-stressed (57 degrees C, 50 min and 60 min) cells of Escherichia coli O157:H7, were enumerated using three media supplemented with 1% sodium pyruvate (NaPyr): plate count agar (PCA), tryptic soy agar (TSA) and phenol red sorbitol agar (PhRSA) using the spread plate method. The medium recovering the greatest numbers of severely heated E. coli O157:H7 was PCA with 1% NaPyr. Recovery of heat stressed E. coli O157:H7 on this medium was significantly higher (P < 0.05) than the two other media with pyruvate: 16.3% (50 min heating) and 0.55% (60 min heating) of the total population was recovered with TSA + 1% NaPyr when compared to those numbers found on PCA + 1% NaPyr. The ability of PhRSA + 1% NaPyr to recover heat-stressed E. coli O157:H7 was similar to that of TSA + 1% NaPyr. Using PhRSA + 1% NaPyr media. 12.9% (50 min heating) and 0.61% (60 min heating) of the total population were recovered when compared with the cells enumerated on PCA + 1% NaPyr. Recovery of the heat-stressed cells using the spread plate method was greater than using pour plate method. Recovery was significantly higher (P < 0.05) on the spread plates for highly stressed E. coli O157:H7(1.2 log) heated for 60 min than on the pour plates. Overall, the populations on the TSA spread and pour plates were low compared with the same heat-stressed cells recovered on media containing pyruvate. The

Media for the detection and enumeration of Bacillus cereus in foods: a review

Bacillus cereus is an established cause of food poisoning in addition to being a troublesome and persistent contaminant, responsible for a variety of spoilage defects in processed foods and dairy products. A range of diagnostic and selective media has been developed to facilitate the detection and enumeration of B. cereus in routine surveillance situations and food poisoning investigations. These media are reviewed with respect to the selective and diagnostic systems they employ, their ability to recover and differentiate the target organism, and their advantages and limitations in particular applications.