Frequency of tetracycline resistance determinant classes among lactose-fermenting coliforms

Abstract

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:

Tetracycline Antibiotics: Mode of Action, Applications, Molecular Biology, and Epidemiology of Bacterial Resistance

“…This was evident from the earliest studies of the distribution of tet genes, when it was found that only 3.5% of the lactose-fermenting coliforms carried two different tet genes (169). Similar results have been found in bacteria isolated from catfish and their environment (59)(60)(61), in Shigella spp.…”

“…and Streptomyces spp. (207) but is uncommon in facultative gramnegative bacteria, especially enteric species (8,60,61,81,115,(168)(169)(170)176). The reason for this is unknown, but a similar situation exists for the carriage of other antibiotic resistance genes (155).…”

Epidemiology of Tetracycline Resistance Determinants in Shigella spp. and Enteroinvasive Escherichia coli : Characterization and Dissemination of tet (A)- 1

“…Molecular techniques such as PCR assays that identify the Tet resistance determinants are a valuable tool for epidemiological studies of resistance, providing important epidemiological information about the transmission of these genes through bacterial populations and the geographical and temporal spread of particular clones and allowing discrimination between clonal spread and horizontal transfer of resistance determinants (1,5,8,11,16,30). Recent studies on gram-negative bacteria have examined the distribution of specific Tet determinants using genotypic detection assays (3,5,7,11,16,21,22,27). However, only one paper (22), which included only 33 strains isolated in Mexico City, Mexico, in 1978 to 1979, has studied the distribution of these determinants in Shigella spp.…”

Genetic basis of tetracycline resistance in urogenital bacteria

“…Currently, there are 13 well-characterized tetracycline resistance determinants representing three different mechanisms of resistance. Five of these determinants, Tet A to Tet F, are found only in gram-negative bacteria (11,12,14), while Tet K, Tet L, Tet N, and Tet P have previously been described only for gram-positive species (1,4,7,23). Tet M has been found in gram-positive (3)(4)(5)(6)20), gram-negative (8,13,20,23), and cell-wall-free species (18,19), whereas Tet 0 has been associated with Streptococcus spp., Enterococcus spp.…”