Predicting fungal growth: the effect of water activity on Aspergillus flavus and related species.

Growth of four species belonging to Aspergillus Section Flavi (A. flavus, A. oryzae, A. parasiticus and A. nomius) was studied at 30 degrees C at ten water activities (aw) between 0.995 and 0.810 adjusted with equal mixtures of glucose and fructose. Colony diameters were measured at intervals and plotted against time. A flexible growth model describing the change in colony diameter (mm) with respect to time was first fitted to the measured growth data and from the fitted curves the maximum colony growth rates were calculated. These values were then fitted with respect to aw to predict colony growth rates at any aw within the range tested. The optimum aw for each species and time to reach a colony diameter of 3 mm were also calculated.

Polymerase chain reaction for detection of Clostridium botulinum types A, B and E in food, soil and infant faeces.

The application of the polymerase chain reaction (PCR) for detection of Clostridium botulinum types A, B and E in foods, environmental and clinical samples was evaluated and compared to the mouse bioassay. Samples inoculated with 10, 100 and 1000 spores of Cl. botulinum types A and B included pasteurized milk, UHT milk, infant formula, infant faeces, meat juice, canned tuna, mushrooms, blood sausage and soil. Clostridium botulinum type E spores were inoculated into fish eggs, canned tuna, picked herring, raw fish and soil at similar levels. Spores were added to 2.5 g of each sample with the exception of soil which was inoculated in 10 g samples. The presence of Cl. botulinum in sample enrichments was determined by both PCR and the bioassay. An overall correlation of 95.6% was observed between PCR results and the mouse bioassay. Of the total of 114 samples tested there was disparity between the mouse bioassay and the PCR in three samples of soil inoculated with 100 type A or E spores and 10 type B spores per 10 g, respectively, and two samples of infant faeces inoculated with 10 type A or B spores per 2.5 g. All of these samples gave negative animal results and positive PCR results.

The effects of modified atmospheres on the growth of psychrotrophic pseudomonads on a surface in a model system.

Atmospheres containing concentrations of CO2 as low as 20% (balance nitrogen) inhibited the growth of Pseudomonas fluorescens and Pseudomonas putida on the surface of buffered Brain Heart Infusion agar plates, pH 6.8, incubated at 5 or 15 degrees C in flexible packages. The modified atmospheres decreased the growth rates and reduced the populations attained at the end of the exponential phase of growth, but had no substantial effect on the lag phase. P. fluorescens was less tolerant of CO2 than P. putida. The inhibitory effect of CO2 increased with its concentration and inhibition was greater at 5 than at 15 degrees C. Growth occurred in packages flushed with 20, 40 and 100% CO2 and 100% N2 at 15 degrees C and 20 and 40% CO2 and 100% N2 at 5 degrees C. The residual O2 concentration in the packages after flushing was 0.2-0.5%. Storage of pseudomonads in CO2 under conditions that prevented growth (e.g., 100% CO2, 5 degrees C) did not cause substantial loss of viability. There was no detectable residual effect of CO2. If cultures were incubated in air after storage for up to 70 days in CO2-containing atmospheres which prevented growth, the subsequent growth curve did not differ noticeably from that observed when plates were incubated in air immediately after inoculation. When cultures in the exponential or stationary phases of growth in modified atmospheres were transferred to air, growth rates increased quickly to rates similar to those observed in air and the final populations observed in air were attained. A reduction in the pH of the medium to 5.5 substantially increased the inhibitory effect of CO2. At 5 degrees C and pH 5.5, substantial growth of P. fluorescens was not observed in any of the CO2 concentrations tested, nor in 40 or 100% CO2 for P. putida.

Influence of modified-atmosphere storage on the growth of uninjured and heat-injured Aeromonas hydrophila.

The growth of uninjured and heat-injured Aeromonas hydrophila incubated at 5 degrees C (22 days) and 30 degrees C (31 h) under air, N2, and CO2 was investigated. At 30 degrees C, the growth patterns of cells on brain heart infusion agar incubated under air and N2 were similar, although slight differences in the lengths of the lag phases and the final populations were detected. The lag phases of cells incubated under air and N2 were substantially longer at 5 degrees C than at 30 degrees C. The population of uninjured A. hydrophila incubated at 5 degrees C under air and N2 remained constant, whereas the number of injured cells declined before the exponential growth phase. Growth at 5 degrees C was enhanced when uninjured and heat-injured A. hydrophila were incubated under N2. At 30 degrees C, cells incubated under CO2 exhibited noticeably longer lag phases and lower growth rates than did cells incubated under air and N2. The viable populations of uninjured and heat-injured cells incubated at 5 degrees C under CO2 declined steadily throughout incubation.

The response of Gluconobacter oxydans to sorbic and benzoic acids.

The minimal inhibitory concentrations (MIC) of sorbic and benzoic acids for Gluconobacter oxydans were 1000 mg/l and 900 mg/l respectively at pH 3.8. A reduction in the pH of the test medium to 3.3 reduced the MIC of both preservatives by about 300 mg/l. When G. oxydans was grown in the presence of sublethal concentrations of sorbic or benzoic acids before the MIC was determined, the MIC of both compounds increased substantially within 1 h. Growth of G. oxydans was modified in several ways by the presence of sorbic acid in the medium. The duration of the lag phase increased and there was a substantial decrease in the viable count during the lag phase in the presence of high concentrations. The generation time increased and the viable count at the end of the logarithmic phase was reduced. At 1 degree C, G. oxydans grew in the absence of sorbic acid but was inactivated by 400 mg sorbic acid/l. At 37 degrees C the viable count of suspensions of G. oxydans decreased in both the absence and presence of sorbic acid. Sorbic acid increased the death rate. Growth of G. oxydans was prevented by eliminating air from culture vessels, combined with the addition of ascorbate to the medium containing 400 mg sorbic acid/l.

The response of Campylobacter jejuni and Campylobacter coli in the Sydney rock oyster (Crassostrea commercialis), during depuration and storage.

Sydney Rock Oysters, when allowed to feed in waters containing approximately 10(4) cfu of Campylobacter cells per ml, concentrated between 10(2) and 10(3) cfu of the organism per g of oyster tissue, within 1 h. When these contaminated oysters were subjected to depuration, they were effectively cleaned in 48 h. The survival of Campylobacter jejuni and Campylobacter coli was also investigated. Oysters contaminated by feeding and injection were processed as half shells and bottled oysters and were held at 3 and 10 degrees C. Half shells were also stored at -20 to -24 degrees C. At all these temperatures the organism survived for periods varying between 8 to 14 days and in oysters contaminated by feeding, the survival was substantially greater. Survival was better at 3 than at 10 degrees C in half-shelled oysters. Campylobacter survived better in bottled oysters than in half shells stored at the same temperature. In frozen half shelled oysters previously contaminated by feeding, the organisms were viable for months. In contaminated unopened oysters stored at 20 and 30 degrees C, C. jejuni and C. coli failed to multiply as expected. They survived for periods varying from 2 to 9 days.

Vibrio cholerae and enteric bacteria in oyster-producing areas of two urban estuaries in Australia.

Three sampling sites in oyster-producing areas of 2 estuaries were monitored at intervals of about 2 weeks for 1 year. Oysters (Crassostrea commercialis), water and sediment were examined for Vibrio cholerae, Escherichia coli and Salmonella. V. cholerae was detected in 20, 30 and 11% of oyster, water and sediment samples respectively. The highest incidence was in the autumn (March-May), with few isolations from July to October. Most isolates were non-O1 serotypes. The presence of V. cholerae and the enteric bacteria appeared to be influenced by different, but perhaps overlapping, sets of factors in these high salinity waters. There was no relationship between rainfall or salinity and the detection of V. cholerae, whereas high counts of E. coli in oysters and the presence of Salmonella were correlated wtih rainfall and, to a lesser degree, reduced salinity. High counts of E. coli were correlated with V. cholerae isolations from water and with the presence of Salmonella. Oysters concentrated E. coli effectively. The counts of E. coli in oysters were 7.3 times higher than those in water. Examination of 8 batches of purified and unpurified oysters indicated that purification reduces the incidence of V. cholerae. However V. cholerae was detected in 3 of 25 market samples of oysters, demonstrating that it can be present in oysters throughout the distribution system. The highest V. cholerae count observed in oysters was 3/g.

Assessment of Cooked Prawns as a Vehicle for Transmission of Viral Disease.

A method has been developed for recovery of viruses from cooked peeled prawns. The method involves elution of viruses from the surface of prawns using a suitable buffer, clarification of the extract by centrifugation and concentration of viruses present by ultracentrifugation. In trials with laboratory-contaminated samples of prawns the method recovered at least 70% of added poliovirus 1. The survival of poliovirus 1 on the surface of cooked peeled prawns was followed during storage at 4-6°C and -20°C for up to 15 d and up to 300 d, respectively. A substantial proportion (22-75%) of added virus remained infective for the periods that this product is usually stored either during transport and distribution or in the home. Thirty retail samples of cooked peeled prawns were examined for presence of viruses infective for man. Viruses were not isolated from any sample.

Demonstration of Viral Contamination of Oysters Responsible for an Outbreak of Viral Gastroenteritis.

Two viruses, echovirus type 8 and a reovirus, were isolated from a batch of oysters responsible for an outbreak of gastroenteritis. Characteristics of the illness, detection of Norwalk virus in the feces of one of the victims and other factors indicated strongly that the illness was due to infection with Norwalk virus. Examination of the implicated oysters and a fecal specimen from a victim failed to provide evidence of the involvement of any other causative agent. Thus laboratory evidence of human enteric virus contamination of a batch of food responsible for a viral illness has been provided.

Rapid radiometric method for detection of Salmonella in foods.

A radiometric method for the detection of Salmonella in foods has been developed which is based on Salmonella poly H agglutinating serum preventing Salmonella from producing 14CO2 from [14C]dulcitol. The method will detect the presence or absence of Salmonella in a product within 30 h compared to 4 to 5 days by routine culture methods. The method has been evaluated against a routine culture method using 58 samples of food. The overall agreement was 91%. Five samples negative for Salmonella by the routine method were positive by the radiometric method. These may have been false positives. However, the routine method may have failed to detect Salmonella due to the presence of large numbers of lactose-fermenting bacteria which hindered isolation of Salmonella colonies on the selective agar plates.