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Using colony hybridization techniques and DNA probes derived from four distinct tetracycline resistance determinants, we have examined the frequency of these determinants among 225 lactose-fermenting coliforms isolated from fecal samples of both humans and animals. The class B, or TnJO-type determinant, occurred most frequently at 73.3%, followed by class A (on RP1) at 21.7%, and class C (on pSC101) at 8%; 3.5% of isolates harbored two of these determinants. Hybridization to class D, carried by plasmid RA1, was not found among any of the isolates. One isolate failed to hybridize to any of the probes and represents a fifth class of determinant. No dramatic differences were observed in the frequencies of these determinants among four populations examined: hospital, urban, rural, and laboratory. At low stringency conditions of hybridization we were able to demonstrate cross-hybridization of determinant A with class C DNA and limited reaction with class B DNA, but no reaction with class D DNA.Tetracycline resistance, commonly specified by plasmids, is widely disseminated among various bacterial species and is expressed at different levels of resistance (11, 12). We and others have used the tetracycline analogs chelocardin and minocycline to demonstrate phenotypic differences among strains carrying tetracycline resistance (12, 13). Such evidence strongly suggested genetic heterogeneity of these determinants, and this was later confirmed by DNA-DNA hybridization with unique probes for three different phenotypic resistance determinants (12). Subsequent evidence has demonstrated an even greater heterogeneity among tetracycline resistance determinants; there appears to be no detectable homology between those found in gram-negative and gram-positive species (3,14). Recently, we have developed a specific probe for the tetracycline determinant classified as class D. Using this and probes for the other three determinants, we examined the frequency of classes A through D among 225 tetracyclineresistant, lactose-fermenting coliforms isolated from 199 fecal samples of human and animal donors. All but one of the isolates hybridized to one or more of the known tetracycline resistance determinants. The results support the uniqueness of the four determinants and demonstrate a marked difference in the frequency of these determinants among fecal coliforms. This study t Present address:

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Oxytetracycfine-resistant (OTcr) and tetracycline-resistant (Tc') Aeromonas hydrophila were isolated commonly from catfish intestinal contents and the water and sediment of catfish culture ponds, but less frequently in market catfish. Isolates demonstrated two resistance phenotypes, Tcr OTcr and Tcs OTcr, when plated directly on to MacConkey agar containing 30 Fig of tetracycline or oxytetracycline per ml. Tcs OTcr isolates expressed Tcr after induction by 1 h of growth in tryptic soy broth containing 1 p.g of tetracycline per ml, Over 90% of the resistant aeromonads hybridized with DNA probes for class A or class E Tcr determinants; class E was twice as prevalent as class A. The distribution of class A and E Tcr determinants varied with the Tcr phenotypes. Prior to induction, 86% of isolates with class A determinants were Tcr as well as OTcr, while only