Antibacterial Activity of Meropenem against Pseudomonas aeruginosa , Including Antibiotic-Induced Morphological Changes and Endotoxin-Liberating Effects

Abstract

The in vitro effects of meropenem on Pseudomonas aeruginosa were examined by studying (i) the inhibitory and bactericidal concentrations of meropenem versus those of imipenem for clinical isolates; (ii) changes in bacterial morphology during in vitro culture; and (iii) release of endotoxin induced by meropenem compared with that induced by other antipseudomonal compounds. Meropenem MIC90 and MBC90 values for 108 clinical isolates were 2 and 4.8 mg/l compared to 4.5 and 9.6 mg/l for imipenem. Morphological studies using phase-contrast and scanning electron microscopy showed that meropenem induced the formation of indeterminate bacterial cell forms at drug concentrations of 1-2.5 mg/l (0.5- to 1.25-fold the MIC), while spheroplasts predominated at drug levels exceeding 5 mg/l (2.5-fold the MIC). Determination of free and EDTA-releasable endotoxin activity by means of the Limulus lysate test showed that both meropenem and imipenem liberated significantly less endotoxin than did ceftazidime. Therefore, although meropenem binds to penicillin-binding proteins (PBPs) 2 and 3 (in contrast to imipenem, which binds to PBP2 only), endotoxin release should not be a cause of concern when treating systemic gram-negative infections with this drug.

Rapid Conversion of Pseudomonas aeruginosa to a Spherical Cell Morphotype Facilitates Tolerance to Carbapenems and Penicillins but Increases Susceptibility to Antimicrobial Peptides

“…These results suggest that spherical cell formation may be an intrinsic developmental response of P. aeruginosa that is induced by antibiotic treatment. While the production of spherical cells by carbapenems and penicillins has been reported previously (7,8,19), our observations show for the first time that spherical cell formation is reversible and occurs readily under physiological conditions, indicating that this response may serve as a genuine mechanism of drug tolerance.…”

“…As an initial approach to understand ␤-lactam tolerance by P. aeruginosa, we monitored P. aeruginosa cells under the microscope following treatment with meropenem, a broad-spectrum carbapenem antibiotic that is commonly used to treat P. aeruginosa infections in humans. It has been noted in previous studies that P. aeruginosa cells switch to a spherical morphology when exposed to meropenem (7,8). We hypothesized that this spherical morphotype may be a viable alternative form of the bacterium that mediates survival in the presence of the antibiotic.…”

“…They are the most widely used group of antibiotics in the world (2) and mediate bacterial killing primarily by inhibiting the enzymes, known as penicillin-binding proteins (PBPs), that catalyze the formation of peptidoglycan cross-links in the bacterial cell wall (3). ␤-Lactam drugs typically exhibit bactericidal activity against susceptible organisms (4), and this is often associated with changes in bacterial morphology and the formation of spherical or filamentous cells that are prone to lysis (3,(5)(6)(7)(8).Interestingly, despite inhibiting cell growth, ␤-lactams have been found to have very little bactericidal activity against P. aeruginosa (9-13). This intrinsic tolerance of ␤-lactams likely accounts for the fact that when P. aeruginosa infections are treated, optimal microbiological outcomes occur when ␤-lactam concentrations are maintained at 4 to 6.6 times the MIC throughout the majority of the dosing period (13).…”

“…They are the most widely used group of antibiotics in the world (2) and mediate bacterial killing primarily by inhibiting the enzymes, known as penicillin-binding proteins (PBPs), that catalyze the formation of peptidoglycan cross-links in the bacterial cell wall (3). ␤-Lactam drugs typically exhibit bactericidal activity against susceptible organisms (4), and this is often associated with changes in bacterial morphology and the formation of spherical or filamentous cells that are prone to lysis (3,(5)(6)(7)(8).…”

Molecular Mechanisms of Sulbactam Antibacterial Activity and Resistance Determinants in Acinetobacter baumannii

“…These phenotypes were not strain specific, as they were also observed in ATCC 17978 (data not shown). It has been well established that meropenem (as well as other ␤-lactam) treatment of other Gram-negative bacterial species such as P. aeruginosa notably results in filamentous cells with distinct bulges at lower concentrations, with spheroplast formation occurring only at much higher concentrations (42). Attempts to evaluate morphological changes in A. baumannii at higher concentrations of meropenem were unsuccessful due to rapid lysis of the cells under these conditions.…”