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Penicillin-binding proteins of Rhodococcus equi: potential role in resistance to imipenem
P Nordmann1, M H Nicolas, L Gutmann
1Laboratoire de Microbiologie, Hôpital Raymond Poincaré, Faculté de Médecine Paris-Ouest, Garches, France.
Abstract:
Rhodococcus equi is a gram-positive coccobacillus which, like other members of the order Actinomycetales, is increasingly reported as an opportunistic pathogen in patients with AIDS. The use of combinations of antibiotics that include imipenem (IMP) has been suggested for the treatment of patients infected with R. equi. An antagonism between IMP, meropenem, cefoxitin, ceftriaxone, moxalactam, and oxacillin and other beta-lactams, such as penicillin, amoxicillin, cephalothin, and ticarcillin, was detected in vitro both on Mueller-Hinton agar and in broth for all 10 IMP-susceptible R. equi strains examined. To study the mechanism of the antagonism between beta-lactams, a mutant with decreased susceptibility to IMP (isolate IpR) was selected in vitro from a susceptible clinical isolate of R. equi (isolate IpS). IpR exhibited decreased susceptibility to IMP, meropenem, cefoxitin, ceftriaxone, moxalactam, and oxacillin but not to penicillin, amoxicillin, cephalothin, or ticarcillin. No beta-lactamase was found in IpS, IpS cultured with antagonistic beta-lactams, or IpR strains. Labeling of penicillin-binding proteins (PBPs) revealed four PBPs with molecular masses of ca. 59, 56, 43, and 26 kDa in IpS. In IpR, PBP 3 disappeared and was replaced by PBP 3a of 40 kDa. The 50% saturation of PBP 3 and PBP 3a by the carbapenems correlated with the MICs of these antibiotics, respectively, for IpS and IpR strains. However, PBP 3a was not detected in IpS when IpS was cultured in the presence of beta-lactams, with which antagonism was observed. The present work describes the PBPs of R. equi and reports that IMP resistance in R. equi is related to an altered PBP pattern.
Insights
Rhodococcus equi infections in AIDS patients can be complicated by antibiotic antagonism. Imipenem resistance in R. equi is linked to changes in penicillin-binding proteins (PBPs), specifically the alteration of PBP 3.
Area of Science:
- Microbiology
- Infectious Diseases
- Pharmacology
Background:
- Rhodococcus equi is an opportunistic pathogen in AIDS patients.
- Antibiotic combinations including imipenem are used for R. equi infections.
- In vitro antagonism between imipenem and other beta-lactams against R. equi has been observed.
Purpose of the Study:
- To investigate the mechanism of beta-lactam antagonism in R. equi.
- To characterize the penicillin-binding proteins (PBPs) of R. equi.
- To determine the relationship between PBP alterations and imipenem resistance.
Main Methods:
- Selection of an imipenem-resistant R. equi mutant (IpR) from a susceptible strain (IpS).
- In vitro susceptibility testing of IpS and IpR to various beta-lactam antibiotics.
- Analysis of penicillin-binding proteins (PBPs) in IpS and IpR using labeling techniques.
- Investigation of PBP expression in IpS exposed to antagonistic beta-lactams.
Main Results:
- Antagonism was detected between imipenem and several other beta-lactams against susceptible R. equi strains.
- The imipenem-resistant mutant (IpR) showed decreased susceptibility to carbapenems and some cephalosporins.
- IpR exhibited altered PBP patterns, with the disappearance of PBP 3 and the appearance of a 40 kDa PBP (PBP 3a).
- No beta-lactamase activity was detected in any of the strains or conditions tested.
Conclusions:
- Imipenem resistance in R. equi is associated with alterations in penicillin-binding proteins.
- The modification of PBP 3 to PBP 3a appears to be a key mechanism for carbapenem resistance in R. equi.
- Understanding these PBP changes is crucial for optimizing antibiotic therapy for R. equi infections.