Search

Two of seven tetracycline-resistant (Tcr) Mycobacterium fortuitum group isolates and six Tcr clinical Streptomyces isolates carried gram-positive Tcr determinants (Tet K and Tet L) and Streptomyces resistance determinants (Otr A, Otr B, and Otr C). This represents the first documentation of the acquisition by mycobacteria of determinants coding for antibiotic resistance and suggests the potential for the spread of antibiotic resistance determinants within mycobacterial species.

Clarithromycin is a potent macrolide that has been used for treating infections with nontuberculous mycobacteria. Pairs of susceptible and resistant Mycobacterium intracellulare strains were obtained from patients with chronic pulmonary M. intracellulare infections undergoing monotherapy with clarithromycin. Nucleotide sequence comparisons of the peptidyltransferase region in 23S rRNAs from parental and resistant strains revealed that in three of six resistant strains, for which the MIC was > 32 micrograms/ml, a single base was mutated (Escherichia coli equivalent, A-2058-->G, C, or U). As the modification of adenine 2058 by dimethylation is a frequent cause of macrolide resistance in a variety of different bacteria, we suggest that mutation of A-2058 confers acquired resistance to clarithromycin in M. intracellulare.

The beta-lactamases obtained from culture supernatants and cell extracts of 26 clinical strains and 5 reference strains of Nocardia farcinica were partially characterized. The enzymes exhibited two patterns on isoelectric focusing (IEF). beta-Lactamases from the majority of the 31 strains (87%) including the 5 reference strains exhibited two major bands with pIs of 4.56 and 4.49. The remaining strains had two similar major bands but with slightly higher pIs. Culture supernatants and cell extracts exhibited identical patterns. The two sets of enzymes were functionally indistinguishable by substrate and inhibitor profiles and lack of inducibility. By disk testing, ampicillin, amoxicillin, ticarcillin, amoxicillin-clavulanic acid, and imipenem were highly synergistic with cefotaxime. The enzymes were primarily penicillinases and hydrolyzed cephalosporins at rates of < or = 12% of those for penicillins. N. farcinica beta-lactamases were susceptible to inhibition by clavulanic acid and BRL 42715, exhibiting 50% inhibitory concentrations of 0.025 to 0.045 micrograms/ml (0.12 to 0.22 microM) and 0.05 to 0.1 micrograms/ml (0.31 to 0.63 microM), respectively, less susceptible to tazobactam, and least susceptible to sulbactam, cloxacillin, and imipenem. The beta-lactamases of N. farcinica are believed to mediate penicillin resistance and may play a secondary role in extended-spectrum cephalosporin resistance. The close similarity among N. farcinica beta-lactamases and their distinct differences from beta-lactamases of other Nocardia species support the taxonomic identity of this species.

Resistance to mercury was evaluated in 356 rapidly growing mycobacteria belonging to eight taxonomic groups. Resistance to inorganic Hg2+ ranged from 0% among the unnamed third biovariant complex of Mycobacterium fortuitum to 83% among M. chelonae-like organisms. With cell extracts and 203Hg(NO3)2 as the substrate, mercuric reductase (HgRe) activity was demonstrable in six of eight taxonomic groups. HgRe activity was inducible and required NADPH or NADH and a thiol donor for optimai activity. Species with HgRe activity were also resistant to organomercurial compounds, including phenylmercuric acetate. Attempts at intraspecies and intragenus transfer of HgRe activity by conjugation or transformation were unsuccessful. Mercury resistance is common in rapidly growing mycobacteria and appears to function via the same inducible enzyme systems already defined in other bacterial species. This system offers potential as a strain marker for epidemiologic investigations and for studying genetic systems in rapidly growing mycobacteria.

Previous studies have demonstrated that Nocardia brasiliensis is susceptible to amoxicillin-clavulanic acid and that its beta-lactamases are inhibited in vitro by clavulanic acid. A cardiac transplant patient with disseminated infection caused by N. brasiliensis was treated with this drug combination with good response, but relapsed while still on therapy. The relapse isolate was found to be identical to the initial isolate by using genomic DNA restriction fragment patterns obtained by pulsed field gel electrophoresis, but it was resistant to amoxicillin-clavulanic acid. On isoelectric focusing, the beta-lactamase from the relapse isolate exhibited a shift in the isoelectric point (pI) of its major band from 5.10 to 5.04 compared with the enzyme from the pretreatment isolate. As determined by using values of the amount of beta-lactamase inhibitor necessary to give 50 +/- 5% inhibition of beta-lactamase-mediated hydrolysis of 50 microM nitrocefin, the beta-lactamase of the relapse isolate was also 200-fold more resistant than the enzyme from the pretreatment isolate to clavulanic acid and was more resistant to sulbactam, tazobactam, cloxacillin, and imipenem. The beta-lactamase of the relapse isolate exhibited a 10-fold decrease in hydrolytic activity for cephaloridine and other hydrolyzable cephalosporins compared with that for nitrocefin. Acquired resistance to amoxicillin-clavulanic acid in this isolate of N. brasiliensis appears to have resulted from a mutational change affecting the inhibitor and active site(s) in the beta-lactamase.

Two-high-level-tetracycline-resistance Moraxella (Branhamella) catarrhalis strains were shown to carry DNA sequences which hybridized with the Tet B probe. The determinant appeared to be located in the chromosome and was nontransferable.

Two closely related beta-lactamases, BRO-1 and BRO-2 (formerly called Ravasio and 1908), are found in Moraxella (Branhamella) catarrhalis. We screened strains of B. catarrhalis recovered in the United States since 1952 and identified the first beta-lactamase-positive isolate in August 1976. The prevalence of the enzymes among 394 clinical isolates from one Texas hospital has averaged 75% since testing began in 1983. Screening of isolates of Moraxella subgenus Moraxella revealed the BRO enzymes in two other human respiratory tract species, M. lacunata and M. nonliquefaciens, beginning in 1978. A different beta-lactamase with a pI of 6.4 predominated in other species of subgenus Moraxella. BRO-2 had a different isoelectric focusing pattern and was produced in lesser amounts than BRO-1, but the two enzymes were indistinguishable by substrate or inhibitor profile. BRO enzymes from B. catarrhalis, M. nonliquefaciens, and M. lacunata could be transferred by conjugation and, for B. catarrhalis, also by transformation to B. catarrhalis. Plasmid bands were demonstrated in 90% of M. nonliquefaciens and in one previously reported strain of B. catarrhalis, but no change in plasmid profiles was seen in beta-lactamase-positive recombinants, supporting previous studies that suggested the beta-lactamase genes are chromosomal.