Synergic post-antibiotic effect of mecillinam, in combination with other beta-lactam antibiotics in relation to morphology and initial killing
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H Hanberger1, L E Nilsson, E Svensson
1Department of Infectious Diseases, University Hospital, Linköping, Sweden.
Abstract:
The synergic in-vitro post-antibiotic effect (PAE) of mecillinam, in combination with either ampicillin, aztreonam, ceftazidime or piperacillin, on a reference strain of Escherichia coli was evaluated by bioluminescence assay of bacterial ATP. Ampicillin, ceftazidime and mecillinam alone induced a concentration dependent PAE (greater than 3 h) on E. coli, whereas aztreonam and piperacillin alone induced a short (less than 1 h) non-dose dependent PAE. At most concentrations, the combination of mecillinam and ampicillin, aztreonam, ceftazidime or piperacillin induced longer PAEs on E. coli than the sum of the individual antibiotics' PAEs. Long PAEs were seen concomitantly with the presence of spheroplasts. In addition to the synergistic PAE, the decrease in colony counts and changes in ATP values after a 2 h exposure to mecillinam, in combination with the other beta-lactam antibiotics, were more prominent than the respective values after exposure to the individual antibiotics. The change in ATP was generally less pronounced than the decrease in colony counts. This could be due to lysis of spheroplasts on agar plates, leading to an over-estimation of the initial killing when assayed by viable counting. Mecillinam, which induced long PAEs on E. coli at almost all concentrations in this study, has a high affinity for penicillin binding protein 2 (PBP 2) and induced spheroplast formation at all concentrations. However, mechanisms other than the affinity for PBP 2 and spheroplast formation are involved in the PAE of beta-lactam antibiotics on Gram-negative bacteria; since the PAE was prolonged when mecillinam was combined with ampicillin, aztreonam, ceftazidime or piperacillin, which bind preferentially to PBP 1 and 3.
Insights
Combining mecillinam with other beta-lactam antibiotics creates a synergistic post-antibiotic effect (PAE) on Escherichia coli. This combination therapy enhances bacterial killing and prolongs the PAE, suggesting improved therapeutic potential.
Area of Science:
- Microbiology
- Pharmacology
- Bacterial Physiology
Background:
- The post-antibiotic effect (PAE) is crucial for understanding antibiotic efficacy.
- Beta-lactam antibiotics exhibit varied PAEs on Gram-negative bacteria like Escherichia coli.
- Mecillinam's unique interaction with penicillin-binding protein 2 (PBP 2) influences its PAE.
Purpose of the Study:
- To evaluate the synergistic in-vitro post-antibiotic effect (PAE) of mecillinam combined with other beta-lactam antibiotics on Escherichia coli.
- To investigate the impact of these combinations on bacterial ATP levels and colony counts.
- To explore the mechanisms underlying the enhanced PAE observed with mecillinam combinations.
Main Methods:
- Bioluminescence assay of bacterial adenosine triphosphate (ATP) to measure PAE.
- In-vitro testing of mecillinam in combination with ampicillin, aztreonam, ceftazidime, and piperacillin.
- Assessment of bacterial colony counts and ATP changes after antibiotic exposure.
Main Results:
- Mecillinam, ampicillin, and ceftazidime induced concentration-dependent PAEs (>3 h) on E. coli, while aztreonam and piperacillin showed short PAEs (<1 h).
- Combinations of mecillinam with other beta-lactams resulted in significantly longer PAEs than the sum of individual effects.
- Synergistic effects were observed, with enhanced bacterial killing and ATP reduction in combination treatments, often correlating with spheroplast formation.
Conclusions:
- Mecillinam exhibits synergistic PAEs when combined with ampicillin, aztreonam, ceftazidime, or piperacillin against Escherichia coli.
- The observed synergistic PAE suggests that mechanisms beyond PBP 2 affinity and spheroplast formation are involved in beta-lactam activity.
- Combination therapy with mecillinam may offer enhanced therapeutic benefits for infections caused by Gram-negative bacteria.