In vitro Susceptibility to β-Lactam Antibiotics and Viability of Neisseria gonorrhoeae Strains Producing Plasmid-Mediated Broad- and Extended-Spectrum β-Lactamases

Neisseria gonorrhoeae plasmids can mediate high-level antimicrobial resistance. The emergence of clinical isolates producing plasmid beta-lactamases that can hydrolyze cephalosporins, the mainstay treatment for gonorrhea, may be a serious threat. In this work, N. gonorrhoeae strains producing plasmid-mediated broad- and extended-spectrum beta-lactamases (ESBLs) were obtained in vitro, and their viability and beta-lactam antibiotic susceptibility were studied. Artificial pbla(TEM-1) and pbla(TEM-20) plasmids were constructed by site-directed mutagenesis from a pbla(TEM-135) plasmid isolated from a clinical isolate. Minimum inhibitory concentration (MIC) values for a series of beta-lactam antibiotics, including benzylpenicillin, ampicillin, cefuroxime, ceftriaxone, cefixime, cefotaxime, cefepime, meropenem, imipenem, and doripenem, were determined. The N. gonorrhoeae strain carrying the pbla(TEM-20) plasmid exhibited a high level of resistance to penicillins and second-fourth-generation cephalosporins (MIC >= 2 mg/L) but not to carbapenems (MIC <= 0.008 mg/L). However, this strain stopped growing after 6 h of culture. The reduction in viability was not associated with loss of the plasmid but can be explained by the presence of the plasmid itself, which requires additional reproduction costs, and to the expression of ESBLs, which can affect the structure of the peptidoglycan layer in the cell membrane. Cell growth was mathematically modeled using the generalized Verhulst equation, and the reduced viability of the plasmid-carrying strains compared to the non-plasmid-carrying strains was confirmed. The cell death kinetics of N. gonorrhoeae strains without the pbla(TEM-20) plasmid in the presence of ceftriaxone can be described by a modified Chick-Watson law. The corresponding kinetics of the N. gonorrhoeae strain carrying the pbla(TEM-20) plasmid reflected several processes: the hydrolysis of ceftriaxone by the TEM-20 beta-lactamase and the growth and gradual death of cells. The demonstrated reduction in the viability of N. gonorrhoeae strains carrying the pbla(TEM-20) plasmid probably explains the absence of clinical isolates of ESBL-producing N. gonorrhoeae.

Automated summary

This study found that Neisseria gonorrhoeae strains carrying plasmids exhibited high-level resistance to β-lactam antibiotics. However, these plasmid-carrying strains showed reduced viability, possibly due to the additional reproduction costs of the plasmid and the expression of ESBLs. This finding may explain the absence of ESBL-producing N. gonorrhoeae clinical isolates.