In vitro selected fluoroquinolone-resistant mutants of Citrobacter freundii: analysis of the quinolone resistance acquisition

Citrobacter keratitis: predisposing factors and clinical characteristics

PURPOSE

To report predisposing factors, clinical presentation, antibiotic sensitivity, and management of Citrobacter-caused infectious keratitis.

METHODS

We retrospectively reviewed the medical records of culture-proven cases of Citrobacter keratitis in a tertiary referral center for 8 years (from January 2012 to September 2020). Demographic data of the patients, predisposing factors, and presenting signs were extracted.

RESULTS

Eighteen cases of microbial keratitis due to Citrobacter spp. were identified. The median age of the patients was 66 years (range: 10-89, interquartile range : 59-81). Thirteen patients were male and 5 were female. Multiple predisposing factors were identified in all eyes, including ocular surface disease (n = 8), previous corneal surgery (n = 6), and history of ocular trauma (n = 6). Five patients were diabetic. Corrected distance visual acuity (CDVA) of patients was light perception (LP) in 8 patients, hand motion (HM) in 7, counting fingers (CF) at 1 m in 1, and CF at 2 m in 2 patients. Thirteen eyes exhibited hypopyon. An area of corneal thinning was observed in 7 eyes (38.9%). Endophthalmitis due to infectious keratitis developed in one patient. In vitro susceptibility testing confirmed high sensitivity to ceftazidime and aminoglycosides. Medical management consisted primarily of topical amikacin (20 mg/ml) combined with topical cefazoline (50 mg/ml) (72.2%). Surgical tectonic procedures were carried out in 7 eyes (38.9%).

CONCLUSION

Citrobacter spp. is a rare cause of bacterial keratitis.Previous keratoplasty and ocular surface problems are important risk factors. The prognosis is not good and surgical tectonic intervention is required in many cases to resolve the corneal infection.

The challenge of efflux-mediated antibiotic resistance in Gram-negative bacteria

The global emergence of multidrug-resistant Gram-negative bacteria is a growing threat to antibiotic therapy. The chromosomally encoded drug efflux mechanisms that are ubiquitous in these bacteria greatly contribute to antibiotic resistance and present a major challenge for antibiotic development. Multidrug pumps, particularly those represented by the clinically relevant AcrAB-TolC and Mex pumps of the resistance-nodulation-division (RND) superfamily, not only mediate intrinsic and acquired multidrug resistance (MDR) but also are involved in other functions, including the bacterial stress response and pathogenicity. Additionally, efflux pumps interact synergistically with other resistance mechanisms (e.g., with the outer membrane permeability barrier) to increase resistance levels. Since the discovery of RND pumps in the early 1990s, remarkable scientific and technological advances have allowed for an in-depth understanding of the structural and biochemical basis, substrate profiles, molecular regulation, and inhibition of MDR pumps. However, the development of clinically useful efflux pump inhibitors and/or new antibiotics that can bypass pump effects continues to be a challenge. Plasmid-borne efflux pump genes (including those for RND pumps) have increasingly been identified. This article highlights the recent progress obtained for organisms of clinical significance, together with methodological considerations for the characterization of MDR pumps.

Partial characterisation of the acrAB locus in two Citrobacter freundii clinical isolates

We studied the mechanisms of resistance to fluoroquinolones in two Citrobacter freundii strains (1.44 and 1.38) isolated from the same patient and belonging to the same clone by pulsed-field gel electrophoresis. This study allowed partial characterisation of the acrA and acrB genes of this microorganism. As previously reported, the two strains showed the same substitutions in the GyrA and ParC proteins (Thr-83-->Ile and Asp-87-->Tyr in GyrA and Ser-83-->Ile in ParC). However, differences were observed in the amount of ciprofloxacin accumulated, with strain 1.38 showing less accumulation. Expression of genes in both strains was analysed using DNA microarrays for Escherichia coli. Ten genes were overexpressed in strain 1.38 compared with strain 1.44, including genes acrA and acrB. Nucleotide similarity between the partially sequenced acrA and acrB genes of C. freundii and E. coli was 80.7% and 85%, respectively. The acrA and acrB genes of C. freundii are similar to those described in E. coli and their overexpression may play an important role in modulating the final minimum inhibitory concentration of fluoroquinolones in collaboration with mutations in the gyrA and parC genes.

Efflux-mediated antimicrobial resistance

Antibiotic resistance continues to plague antimicrobial chemotherapy of infectious disease. And while true biocide resistance is as yet unrealized, in vitro and in vivo episodes of reduced biocide susceptibility are common and the history of antibiotic resistance should not be ignored in the development and use of biocidal agents. Efflux mechanisms of resistance, both drug specific and multidrug, are important determinants of intrinsic and/or acquired resistance to these antimicrobials, with some accommodating both antibiotics and biocides. This latter raises the spectre (as yet generally unrealized) of biocide selection of multiple antibiotic-resistant organisms. Multidrug efflux mechanisms are broadly conserved in bacteria, are almost invariably chromosome-encoded and their expression in many instances results from mutations in regulatory genes. In contrast, drug-specific efflux mechanisms are generally encoded by plasmids and/or other mobile genetic elements (transposons, integrons) that carry additional resistance genes, and so their ready acquisition is compounded by their association with multidrug resistance. While there is some support for the latter efflux systems arising from efflux determinants of self-protection in antibiotic-producing Streptomyces spp. and, thus, intended as drug exporters, increasingly, chromosomal multidrug efflux determinants, at least in Gram-negative bacteria, appear not to be intended as drug exporters but as exporters with, perhaps, a variety of other roles in bacterial cells. Still, given the clinical significance of multidrug (and drug-specific) exporters, efflux must be considered in formulating strategies/approaches to treating drug-resistant infections, both in the development of new agents, for example, less impacted by efflux and in targeting efflux directly with efflux inhibitors.

Efflux-mediated multiresistance in Gram-negative bacteria

Multiresistance in Gram-negative pathogens, particularly Pseudomonas aeruginosa, Stenotrophomonas maltophilia, Acinetobacter spp. and the Enterobacteriaceae, is a significant problem in medicine today. While multiple mechanisms often contribute to multiresistance, a broadly distributed family of three-component multidrug efflux systems is an increasingly recognised determinant of both intrinsic and acquired multiresistance in these organisms. Homologues of these efflux systems are also readily identifiable in the genome sequences of a wide range of Gram-negative organisms, pathogens and non-pathogens alike, where they probably promote efflux-mediated resistance to multiple antimicrobials. Significantly, these systems often accommodate biocides, raising the spectre of biocide-mediated selection of multiresistance in Gram-negative pathogens. While there is some debate as to the natural function of these efflux systems, only some of which are inducible by their antimicrobial substrates, their contribution to resistance in a variety of pathogens nonetheless makes them reasonable targets for therapeutic intervention. Indeed, given the incredible chemical diversity of substrates accommodated by these efflux systems, it is likely that many novel or yet to be discovered antimicrobials will themselves be efflux substrates and, as such, efflux inhibitors may become an important component of Gram-negative antimicrobial therapy.

The role of multidrug efflux pumps in the antibiotic resistance of Pseudomonas aeruginosa and other gram-negative bacteria. Insights from the Society of Infectious Diseases Pharmacists

Gram-negative bacteria remain clinically important pathogens in both hospital and community settings. Recent research indicates that efflux pumps play a prominent role in the multidrug resistance of Pseudomonas aeruginosa and many other gram-negative bacteria. Four multidrug efflux pump systems have been well characterized in P. aeruginosa: MexA-MexB-OprM, MexC-MexD-OprJ, MexE-MexF-OprN, and MexX-MexY-OprM. These efflux pumps have different substrate specificities, and their production and activity can be increased by many factors commonly present in infections (e.g., high inocula of bacteria, low pH, and stationary-phase growth). Moreover, fluoroquinolone antibiotics can commonly select mutants that constitutively overproduce Mex-Opr efflux pump systems. Based on most recent studies, the prevalence of efflux pump overproduction in clinical strains of P. aeruginosa may range from 14-75%. The best treatment for infections caused by bacteria that overproduce efflux pumps is unknown, but pharmacodynamic optimization of antibiotics and the use of antibiotic combinations that are substrates for different pump systems may represent reasonable strategies until more data are available.