Drug resistance and distribution of R factors in fecal Escherichia coli strains isolated from domestic fowls of different weeks of ages (Department of Animal Husbandry)

飼料の安全性の確保および品質の改善に関する法律の実施(1977年1月)によって家畜飼料への抗菌剤等の添加が厳しく規制された。法実施初年度に, 薬剤耐性菌及びR因子保有耐性菌の検出率がどのように変るかを調べるため, 発育期を異にする幼すう(1∿4週齢), 中すう(4∿9週齢), 大すう(9∿15週齢)及び産卵鶏(6ケ月齢以上)の糞便由来大腸菌各250株, 計1,000株を対象に, APC, SM, TC, CP, KM及びSAの6抗菌剤を用いて実験を行なった。得られた成績は以下の通りである。1.耐性菌の検出率は, 中すう, 幼すう, 大すう, 産卵鶏の順に高く, それぞれ96.0,94.4,79.6,60.4%であった。週齢の若いものがむしろ耐性菌保有率が高い傾向にあり, 全体としては82.6%が耐性菌であった。2.何れの鶏群においても主に検出される耐性型はTC, SA, SMの単独型またはそれらの組合わせ型である。これら3剤を主体とした多剤耐性菌の検出率は幼すうで79.6%で最も高く, 次いで中すう78.4,大すう72.0,産卵鶏37.2の順で, 発育と共に検出率が低下の傾向にあり, 平均耐性型はそれぞれ2.3,2.2,2.0及び1.2剤であった。3.R因子の検出率は幼すう21.6%, 中すう14.2%, 大すう41.2,産卵鶏35.8,全体で平均26.8%を示し, 発育との関連性は特に認められない。しかし耐性菌が若いものに多く検出されたに対し, R因子はむしろ発育の進んだ鶏群に多く認められた。4.法実施前に実施したニワトリ(大すう及び産卵鶏)の耐性菌及びR因子保有菌の検出率, 94.6%及び36.4%と今回の成績を比較し法実施による影響をみると, 耐性菌は減少しているが, R因子保有菌の検出率には差がでてない。しかし耐性菌においても, その耐性型と飼料添加が不許可になった薬剤との関連性は特に認められなかった。Legislative controls of the use of antimicrobial drugs in animal feeds in Japan became effective in January 1977. To confirm the effect of the regulation, incidences of drug resistant and R factors-carrying fecal E. coli strains isolated from young- (1∿4 weeks of age) and old-chicks (4∿9 weeks), pullets (9∿15 weeks) and laying hens (6 months or more) were compared. A total of 1,000 strains, 250 strains from each group, were tested using 6 kinds of drugs : APC, SM, TC, CP, KM and SA. The results are summarized as follows : 1. The isolation frequencies of drug resistant strains were 94.4,96.0,79.6 and 60.4% in young-, old-chicks, pullets and laying hens, respectively. The higher frequencies were obtained in isolates from younger birds than those from adult ones. An average percentage of resistance in all isolates was 82.6%. 2. In any group, the most frequently isolated resistance patterns were TC, SA, SM and combinations thereof. The multiple drug resistant strains, their patterns were mainly composed of these 3 drugs, were most frequently encountered in isolates from young-chicks (79.6%), followed by old-chicks (78.4%), pullets (72.0%) and laying hens (37.2%). The likelihood of incidence of multiple drug resistance appeared to decrease with the age of the fowls. 3. Of all resistant strains, 26.8% were found to carry R factors. The highest isolation frequency of R factors-carrying strains was obtained in the resistant strains of pullets, being 41.2%. Following sequence of occurrence was in laying hens (35.8%), young-chicks (21.6%) and old-chicks (14.2%). Though, there are no correlation between isolation frequency of R factors and age of birds from which the test strains originated, the R factors-carrying strains seem to be frequently found in the strains isolated from older birds than those from younger birds. 4. The isolation frequencies of resistant and R factors-carrying strains among chicken isolates, which were reported by the present author in 1974 before the legislative control was enforced, were compared with those obtained in the present experiment. In regard to the isolation frequency of resistant strains, 95.4% which obtained formerly decreased to 82.6%, but their resistance patterns were independent of the drugs added to feeds. The effect of legislative control on persistence of drug resistance was discussed