Imipenem resistance in Pseudomonas aeruginosa is considered to be associated with loss of the porin OprD combined with activity of chromosomal beta-lactamase (AmpC), while overexpression of multidrug efflux pumps is considered to confer meropenem resistance. Carbapenem resistance can also result from production of metallo-beta-lactamases. Transcription of oprD and efflux pump genes mexB, mexY and mexF was analysed in 23 clinical isolates of P. aeruginosa by quantitative RT-PCR. oprD was sequenced in all, and mexR, regulator of efflux pump MexAB-OprM, in selected isolates. Four isolates that were imipenem susceptible had significant reduction of oprD mRNA and presence of oprD mutations causing frameshift or translational stop. In strains only resistant to imipenem no significant difference in transcription of oprD was observed between low-level and high-level resistant isolates. The differences could not be explained by either pattern of oprD mutations. Increased transcription of mexB generally correlated well with meropenem resistance. One high-level meropenem-resistant isolate showed no significant change in mexB mRNA, but sequencing confirmed presence of a nalB mutation. Furthermore, one meropenem-susceptible isolate showed significant increase in mexB transcription, but no mexR mutations. In summary, our findings indicate that the resistance patterns observed cannot be fully explained by the currently described carbapenem resistance mechanisms.

Mutations in the gyrA gene contribute considerably to quinolone resistance in Escherichia coli. Mechanisms for quinolone resistance in anaerobic bacteria are less well studied. The Bacteroides fragilis group are the anaerobic organisms most frequently isolated from patients with bacteremia and intraabdominal infections. Forty-four clinafloxacin-resistant and-susceptible fecal and clinical isolates of the B. fragilis group (eight Bacteroides fragilis, three Bacteroides ovatus, five Bacteroides thetaiotaomicron, six Bacteroides uniformis, and 22 Bacteroides vulgatus) and six ATCC strains of the B. fragilis group were analyzed as follows: (i) determination of susceptibility to ciprofloxacin, levofloxacin, moxifloxacin, and clinafloxacin by the agar dilution method and (ii) sequencing of the gyrA quinolone resistance-determining region (QRDR) located between amino acid residues equivalent to Ala-67 through Gln-106 in E. coli. Amino acid substitutions were found at hotspots at positions 82 (n ‫؍‬ 15) and 86 (n ‫؍‬ 8). Strains with Ser82Leu substitutions (n ‫؍‬ 13) were highly resistant to all quinolones tested. Mutations in other positions of gyrA were also frequently found in quinolone-resistant and -susceptible isolates. Eight clinical strains that lacked mutations in their QRDR were susceptible to at least two of the quinolones tested. Although newer quinolones have good antimicrobial activity against the B. fragilis group, quinolone resistance in B. fragilis strains can be readily selected in vivo. Mutational events in the QRDR of gyrA seem to contribute to quinolone resistance in Bacteroides species.

High-level quinolone resistance in Enterococcus faeciumwas associated with mutations in both gyrA andparC genes in 10 of 11 resistant strains. One low-level resistant strain without such mutations may instead possess an efflux mechanism or alterations in the other subunits of the gyrase or topoisomerase IV genes. These findings are similar to those for other gram-positive bacteria, such as Enterococcus faecalis.

BACKGROUND AND OBJECTIVESMethicillin-resistant Staphylococcus aureus (MRSA) emerged in 1960 and was a problem confined largely to the healthcare setting, or hospital-associated MRSA (HA-MRSA). In the 1990s, community-associated MRSA (CA-MRSA) infections appeared. In Saudi Arabia, the prevalence of MRSA has increased in the past ten years and severe community-acquired infection has been reported. Our objective was to investigate the prevalence of MRSA and their antibiotic susceptibilities in the western region of Saudi Arabia.DESIGN AND SETTINGA retrospective review of the medical records of 186 S aureus infected patients diagnosed from November 2009 through October 2010.METHODSS aureus was Identified based on Gram stain, catalase and coagulase tests. Susceptibility testing was performed using antibiotic discs and the VITEK 2 system.RESULTSMRSA was isolated in 39.5% of the specimens. The isolates were commonly associated with wound, skin, and soft tissue infections (87.3%). The prevalence of MRSA was highest among patients who were 56 years old or older (52.2%). CA-MRSA infections represented 31.5% of community S aureus infections, while HA-MRSA accounted 52.6% of hospital S aureus (P=.0029). All MRSA isolates in our study were susceptible to vancomycin, linozolid and teicoplanin. However, multi-resistance was observed in 29.1% of the isolates and was significantly higher among HA-MRSA (P=.03).CONCLUSIONSThe prevalence of MRSA was 39.5%, and infection was commonly associated with wound, skin, and soft tissue infections. MRSA was more prevalent in hospitals and among older patients. All MRSA susceptible to vancomycin, linozolid and teicoplanin.

Enterococcus faecium has six penicillin-binding proteins (PBP), where PBP5 seems to be the main target for beta-lactam antibiotics. The PBP profiles of three imipenem-resistant, ampicillin-sensitive E. faecium strains, isolated from the same patient, were studied using biotinylated ampicillin and chemiluminescence detection. Imipenem resistance in these strains was found to be associated with hyperproduction of PBP5 compared to the ampicillin- and imipenem-susceptible strain ATCC 19434. PBP5 in the imipenem-resistant strains (S1, B2) exhibited a selectively decreased affinity for imipenem. An 854 bp DNA fragment, corresponding to the penicillin-binding domain of pbp5fm, was studied in the resistant strains and the reference strain. Four amino acid substitutions were observed in the resistant strains compared to the susceptible one. The contribution of these substitutions to the increased production of PBP5 in these strains is unclear since the substitution was observed also in a strain without increased production of PBP5. Our results suggest that the moderate imipenem resistance observed in these strains is associated with increased production of PBP5 with relatively decreased affinity for imipenem, and that evolution of imipenem resistance in E. faecium is dinstinct from that of the other beta-lactams such as ampicillin.