Drug resistance of staphylococci. VI. Genetic determinant for chloramphenicol resistance

Mechanism of Chloramphenicol Resistance in Staphylococcus epidermidis

The mechanism of chloramphenicol resistance in several multiple-resistant Staphylococcus epidermidis strains has been studied and shown to be due to the presence of the enzyme, chloramphenicol acetyltransferase. As with S. aureus, the inactivating enzyme in S. epidermidis appears to be the product of a structural gene on the chloramphenicol plasmid because resistance and enzyme activity are concurcurrently lost after growth in acridine orange or at elevated temperatures. The synthesis of chloramphenicol acetyltransferase in S. epidermidis has been compared with the function of a similar enzyme in chloramphenicol-resistant S. aureus with the conclusion that the kinetics of induction, products of the reaction, and general properties of the enzymes are identical. The chloramphenicol acetylating enzyme from S. epidermidis has been purified to a state of homogeneity and compared with the analogous purified S. aureus enzyme. Both purified preparations consist of native enzymes with molecular weights of 80,000, and evidence is presented that is consistent with their being made up of four identical subunits of 20,000 each. The two staphylococcal enzymes are identical with respect to pH optimum, apparent affinity (K(m)) for chloramphenicol, heat denaturation, and immunological reactivity, but they differ in electrophoretic mobility, chromatographic behavior, substrate specificity, and sensitivity to inhibition by mercuric ion.

R plasmids in Corynebacterium xerosis strains

Plasmids coding for resistance to chloramphenicol, erythromycin, kanamycin, streptomycin, and tetracycline have been found in strains of Corynebacterium xerosis isolated from patients with otitis media.

Inducible high resistance to colistin in Proteus strains

Wild-type Proteus strains are usually resistant to colistin. We have observed an increase in the level of colistin resistance in four clinical isolates of P. vulgaris, P. morganii, and P. rettgeri by prior treatment with subinhibitory concentrations of the drug. The enhanced resistance in these strains was lost after cultivating them in colistin-free medium, indicating that the enhanced resistance in the Proteus group is inducible.

Antibacterial and inducer activities for tetracycline resistance by its derivates and analogues

Antibacterial and inducer activities of thirteen TC derivates were investigated for tetracycline (TC) resistance in staphylococcus aureus. Four compounds of the TC derivatives were not able to induce the resistance to TC in Staphylococcus aureus MS3937 rms7 (TC) + harboring an inducible TC resistance determinant located on a plasmid. The 12a-hydroxy position seems to be essential for the inducer activity.

Isolation of the rec mutants in Staphylococcus aureus

A histidine auxotroph (his-) of Staphylococcus aureus MS3937 and mutants sensitive to ultraviolet (UV) irradiation were obtained. The UV-sensitive mutants were found also to be sensitive to N-methyl-N'-nitro-N-nitrosoguanidine and mitomycin C, and their sensitivity was accounted for by a defect in deoxyribonucleic acid repair. Transduction of either chromosomal or plasmid markers to UV-sensitive mutants showed that these staphylococcus mutants are of the recA (reckless) type mutants as reported in Escherichia coli and Salmonella typhimurium; therefore the UV-sensitive mutants (uvr-) were renamed recombination-deficient mutants (rec-). The biochemical and genetic properties of these mutants are described, and their usefulness for studies of staphylococcal plasmids is discussed.

New type of R factors incapable of inactivating chloramphenicol

Four R factors conferring chloramphenicol (CM) resistance were isolated from Escherichia coli strains of clinical origin. Strains carrying the factors were found to be incapable of inactivating the drug in the presence of acetyl coenzyme A. E. coli W3630 carrying R(70), one of these factors, became sensitive to CM after treatment with glycine, indicating that the spheroplasts of W3630 R(70) (+) were sensitive to the drug and suggesting that the cell membrane is important for CM resistance. The observation that cell-free protein synthesis in W3630 R(70) (+) was inhibited by CM is also compatible with a decrease in permeability. CM resistance in W3630 R(70) (+) appeared to be inducible, because (i) preincubation with subinhibitory concentrations of CM prevented the prolonged lag noted for growth in the presence of 25 mug of CM per ml, and (ii) the preincubation effect was lost after overnight growth in CM-free medium. By contrast, E. coli W3630 cml(+), in which the resistance determinant is integrated into the chromosome, was capable of rapid inactivation of CM. E. coli W3630 cml(+) R(70) (+), which contains the proposed permeability determinant (episomal) as well as levels of the inactivating enzyme (chromosomal) that are comparable with W3630 cml(+), was capable of brief inactivation of CM when inoculated into drug-containing medium. The absence of continued inactivation on more prolonged incubation favors the hypothesis that the R(70) factor inhibited further penetration of CM and that this property possesses the characteristics of induction.

Kinetics of induction and purification of chloramphenicol acetyltransferase from chloramphenicol-resistant Staphylococcus aureus

Plasmid-mediated chloramphenicol resistance in Staphylococcus aureus has been shown to involve acetylation of chloramphenicol by an enzyme induced by growth in the presence of the antibiotic and certain analogues. Analysis of the kinetics of induction has been complicated by (i) the intrinsic inhibitory effects of chloramphenicol on induced enzyme synthesis and (ii) the rapid disappearance of inducer after synthesis of the acetylating enzyme. The compound related to d-threo chloramphenicol which lacks a C(3) hydroxyl substituent (3-deoxychloramphenicol) is a potent inducer of chloramphenicol acetyltransferase but is ineffective as an antibiotic and is not a substrate for the enzyme. The availability of such a "gratuitous" inducer has simplified an analysis of the kinetics of induction of chloramphenicol acetyltransferase. The enzyme from induced bacteria has been purified to homogeneity and has been compared with the analogous enzyme present in E. coli which harbors a resistance transfer factor with the chloramphenicol resistance determinant.