Since the effects of temperature and the formation of volatile basic nitrogen (VBN) on the growth of Listeria monocytogenes in skim milk under aerobic and microaerobic conditions are unknown, a series of experiments were designed to investigate these problems with strain VTU 206 goat. At 10, 20 and 30°C, though the population of the organisms reached a magunitude of 107/ml, little change in pH or VBN was observed. The mean generation times at 30°C under aerobic and microaerobic (O: CO2: N =4.9: 9.9: 85.2) conditions were 48 and 54 min, respectively. At 20°C, the mean generation times under aerobic and microaerobic conditions were 108 and 120 min, respectively. At 10°C, the mean generation times under aerobic and microaerobic conditions were 324 and 360 min, respectively. The Q10 values of the growth of the organisms at 20-30°C and 10-20°C under aerobic and microaerobic conditions were 2.25, 2.22 and 2.96, 3.00, respectively.
From May, 1987 to May, 1989, raw fish and meat samples obtained from retail shops in Hyogo Prefecture were examined for the contamination with Staphylococcus aureus. The isolates were tested for some biological characteristics and enterotoxigenicity. S. aureus was isolated from 148 (44.7%) of 331 fish and 125 (59.5%) of 210 meat samples. It was found in 19 (25.3%) of 75 scad, 35 (34.7%) of 101 sardine, nine (28.1%) of 32 mackerel, 43 (71.7%) of 60 saury, 39 (78.0%) of 50 squid, three (23.1%) of 13 oyster, 102 (92.7%) of 110 chicken, 13 (26.0%) of 50 pork, and 10 (20.0%) of 50 beef samples. Two hundred and thirty (82.1%) of 280 fish isolates and 119 (95.2%) of 125 meat isolates were biotyped. Most of them were biotype B or C. Two hundred and two (72.1%) of the 280 fish isolates and 50 (40.0%) of the 125 meat isolates were typable with the human phages. Most of the fish isolates belonged to mixed groups, group III and group II. Most meat isolates belonged to group III. Eighty (94.1%) of 85 fish isolates and 44 (64.7%) of 68 meat isolates were typable with the eight coagulase-typing antisera. The most predominant type among the fish isolates was type VII, followed by types III, II, I and VIII; that among the meat isolates was type VII, followed by types VIII, III, VI, II and V. Nineteen (22.4%) of the 85 fish isolates and 38 (30.4%) of the 125 meat isolates produced enterotoxin. The most predominant enterotoxin type produced by the fish isolates was B, followed-by A, AB and AD; that produced by the meat isolates was B, followed by C, A and AB. Forty-seven (55.3%) of the 85 fish isolates and 39 (31.2%) of the 125 meat isolates were resistant to one or more of six antibiotics. The most predominant resistance pattern among the fish isolates were PC; that of the meat isolates were LCM/EM and PC.
Among 16 different enzymes measured by fluorogenic assay, the arylamidase activity of Salmonella was the highest at pH 7.5. The arylamidase activities of raw foods were, however, about five-times higher than that of Salmonella at pH 7.5. Six different arylamidases were examined for their optimal pH values; the alanine-arylamidase activity of Salmonella was higher than those of raw pork, raw chicken, and raw beef at pH 9 and pH 10. Dulcitol-malachite green-novobiocin (DMN) medium containing 0.01% malachite green and 1μg/ml novobiocin was effective for enrichment of Salmonella. In the presence of 5mM EDTA, the alanine-arylamidase activity increased to by 3.8 times that measured without EDTA. Although E. aerogenes grew and exibited a high alanine-arylamidase activity at an initial cell number of 3×103, 1-10 cells of Salmonella were detected by measuring the enzyme activity after cultivation in DMN medium at 30°C for 22 hr with shaking. The activity increased by increasing the initial cell number of Salmonella.
The Oxoid Salmonella Rapid Test (OSRT) that utilizes a selective enrichment motility technique to detect Salmonella in foods was evaluated with pure cultures and naturally contaminated raw meat. All motile Salmonella strains tested by the OSRT gave positive results Non-motile Salmonella strain gave negative results. None of hydrogen sulfate-positive bacteria such as Citrobacter freundii, Edwardsiella tarda and Proteus spp gave a positive results. OSRT was compared with the standard culture method for detection of Salmonella in 77 samples of naturally contaminated chicken meat. Twenty-nine (37.6%) of these samples were positive by OSRT and 30 (39.0%) by the culture method. OSRT was found to be a rapid screening method for detection of Salmonella in food.