Teruyoshi Arai’s research while affiliated with Tokyo Metropolitan Institute of Public Health and other places

Between 1986 and 1996, bacterial contamination of 1, 445 commercial raw fish and shellfish in Tama, Tokyo was investigated. Each year, the median of standard plate counts decreased, whereas that of the coliform group was nearly constant. Vibrio parahaemolyticus was detected in 71 (4.9%) samples, and high contamination was observed in shellfish such as ark shell (25 samples). Staphylococcus aureus was also detected in 77 (5.3%) samples, especially tuna (24 samples). In addition, Bacillus cereus was found in 25 (1.7%) samples, but no Salmonella was detected.By introducing enriched cultures for S. aureus, the detection ratio markedly increased, and marked contamination in tuna was observed. In the testing of coagulase and enterotoxin on the 105 strains of S. aureus, all isolates were typable to 8 different kinds of coagulase type, but enterotoxigenicity was confirmed in only 29 strains.

Thermostable direct hemolysin (TDH) produced by Vibrio parahaemolyticus has been supposed to be the greatest causative factor for the enteropathogenicity. However, there have been no reports concerning the existence of TDH in any foods. We tried to detect it in some foods which are well known as the causative foodstuffs of this foodborn disease, using the method of the human red blood cell lysis system. Fresh shrimp showed the lytic activity only when the strain was inoculated. Shirasu-boshi (salted, small and half-dried sardines) already showed the lytic activity without the strain, and increased activity was observed after the inoculation. The lytic activity of fresh oyster was not changed significantly after the inoculation. The pre-incubation with trypsin or pancreatin for 1 hr at 37°C increased the lytic activity in fresh shrimp. These findings auggest that TDH is produced not only in some foods, but also in our small intestine by V. parahaemolyticus.

The antibacterial actions of soy sauce, sawa-wasabi, ginger and garlic on the viability of Vibrio parahaemolyticus were examined mainly at 5°C. Sawa-wasabi acted most strongly and rapidly of these spices, only in soy sauce, and not alone. Allylisothiocyanate, the main pungent component of sawa-wasabi had the same action as sawa-wasabi. Ginger had a strong inhibitory action by itself, but showed no synergy with soy sauce. Garlic did not exert its action even in soy sauce. The antibacterial mechanism of soy sauce is suggested to be due at least to the co-operation of its pH (4.5-4.7), Aw (below 0.9) and the content of sodium chloride (10-18%).

The heat resistance of L. monocytogenes was experimentally examined by heating ground pork samples at 63°C for 30 minutes which is the standard pasteurization procedure for cooked meat products in Japan.Ground pork samples with and without curing salt addition were inoculated with 102/g or 107/g of serotype 1/2b, 1/2c or 4b strain and heated at 63°C for 30 minutes. Although a slight difference was observed in the heat resistance of L. monocytogenes inoculated at the level of 102/g depending on the strain and the presence or absence of curing salt, it perished within 4 minutes. In samples inoculated with 107/g, the bacterium was detected up to 5 minutes after heating regardless of the strain type. The 4b strain was detected at 10 minutes after heating in the samples containing the curing salts and the 1/2c strain at 20 minutes after heating in the samples without the salt addition.However, none of the strains survived after 30 minutes heating either with or without salt addition, indicating that the standard heating requirement is sufficient for pasteurization of L. monocytogenes.

スモークサーモンの原材料の凍結紅サケおよび最終製品を対象にリステリア属菌の汚染調査を行った. その結果, 調査した凍結紅サケ60例のすべてが陰性であったが, 製品では76例中23例 (30.3%) が陽性であった. 陽性例の12例からL. monocytogenesが検出され, このほかL. welshimeri, L. innocua, L. seeligeriが認められた. 分離されたL. monocytogenesの血清型は1/2a, 1/2b, 1/2c, 3a, 3b, 4bの6型に型別された.実験的に1g当たり102レベルにL. monocytogenesを接種したスモークサーモンを好気または嫌気包装して2および10℃に保存したところ, 好気包装では2℃保存で10日目に104～105レベル, 20日目には108レベルとなり, 10℃保存で5日目に107～108レベルに達した. 嫌気包装では好気包装と比較して若干発育が抑制される傾向が認められた. 原材料サケおよび最終製品のスモークサーモンを-20℃で保存した場合は, 実験的に接種した104～105レベルのL. monocytogenesはいずれも6カ月後も1オーダ程度の菌数減少にとどまった.

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.

Bainiku-ekisu (Japanese Apricot Fruit Extracts) is a traditional drug in Japan and its main components were shown to be citric acid and malic acid (32% and 11% on average) by HPLC analysis of 9 commercial products. The antibacterial action of Bainiku-ekisu was investigated in vitro against one of the causative bacteria of food-borne infection, Vibrio parahaemolyticus. Addition of both Bainiku-ekisu (1%) and cholic acid (0.2%) to 2% polypepton medium with sodium chloride at about pH 7.0 decreased the viability of V. parahaemolyticus remarkably, although Bainiku-ekisu did not show the activity by itself. Citric acid is considered to be responsible for this antibacterial action with cholic acid, because we confirmed the effectiveness of citric acid but not of malic acid. The strong acidity of Bainiku-ekisu was enough to change the pH of a model intestinal solution (pH 8.03) to 6, even at 1% concentration.Based on these results, the inhibitory effect of Bainiku-ekisu on Gram-negative and food-borne infectious bacteria in vivo is discussed.

We conducted a survey during the period from 1974 to 1976, to determine the distribution of Plesiomonas shigelloides in human faeces, the intestinal contents of cattle, swine, poultry, dogs, cats, fresh water fish, and river water and sludge from wet riverbeds in the vicinity of Tokyo. Isolation of the organisms was performed by using Salmonella-Shigella (SS) agar and deoxycholate-hydrogensulphide-lactose (DHL) agar plates. P. shigelloides was isolated from 3 (0·0078%) of 38454 healthy Tokyoites, 37 (3·8%) of 967 dogs, 40 (10·3%) of 389 cats, 25 (10·2%) of 246 fresh water fish, 64 (12·8%) of 497 river water samples, and 2 of 19 (10·5%) sludge samples. Of 302 strains isolated, from dogs, cats, fresh water fish, river water and healthy carriers, 196 were typed to 50 serovars. Most of the serovars were found to be similar to strains isolated from patients with gastroenteritis due to P. shigelloides .