Failure of routine susceptibility tests to detect imipenem resistance among strains of methicillin-resistant Staphylococcus aureus

Methicillin-Resistant Staphylococcus aureus Isolated From Various Types of Hospital Infections in Pediatrics: Panton-Valentine Leukocidin, Staphylococcal Chromosomal Cassette mec SCCmec Phenotypes and Antibiotic Resistance Properties

Background:

Staphylococcus aureus has long been considered as a major pathogen of hospital infections.

Objectives:

The present investigation was carried out to study the distribution of Staphylococcal Chromosomal Cassette mec (SCCmec) types, Panton-Valentine Leukocidin (PVL) gene and antibiotic resistance properties of Methicillin Resistant Staphylococcus aureus (MRSA) strains isolated from various types of infections found in Iranian pediatric patients.

Patients and Methods:

Two-hundred and fifty-five clinical specimens were collected from four major provinces of Iran. Samples were cultured and the MRSA strains were subjected to Polymerase Chain Reaction (PCR). The patterns of antibiotic resistance were determined using the disk diffusion method.

Results:

Seventy-four out of 255 (29.01%) clinical samples were positive for MRSA. Of the 74 MRSA strains, 47 (63.51%) were PVL positive. The clinical samples of respiratory tract infections (36.36%), those from the Shiraz province (37.87%) and samples collected during the summer season (56.48%) were the most commonly infected samples. The most commonly detected antibiotic resistance genes were tetK (89.18%), mecA (71.62%), msrA (56.75%) and tetM (54.05%). Methicillin Resistant Staphylococcus aureus had the highest levels of resistance against penicillin (100%), tetracycline (98.64%), ampicillin (93.24%) and oxacillin (93.24%). The most commonly detected SCCmec types in the MRSA strains were type V (18.91%) and III (17.56%).

Conclusions:

Regular surveillance of hospital-associated infections and monitoring of the antibiotic sensitivity patterns are required to reduce the prevalence of MRSA. We recommend initial management of children affected by MRSA with imipenem, lincomycin and cephalothin prescriptions.

Nosocomial Infection Caused by Antibiotic-Resistant Organisms in the Intensive-Care Unit

Resistance to antimicrobial agents is an evolving process, driven by the selective pressure of heavy antibiotic use in individuals living in close proximity to others. The intensive care unit (ICU), crowded with debilitated patients who are receiving broad-spectrum antibiotics and being cared for by busy physicians, nurses, and technicians, serves as an ideal environment for the emergence of antibiotic resistance. Problem pathogens presently include multiply resistant gram-negative bacilli, methicillin-resistantStaphylococcus aureus, and the recently emerged vancomycin-resistant enterococci. The prevention of antimicrobial resistance in ICUs should focus on recognition via routine unit-based sur veillance, improved compliance with handwashing and barrier precautions, and antibiotic-use policies tailored to individual units within hospitals.

Meticillin-resistant Staphylococcus aureus isolated from Iranian hospitals: virulence factors and antibiotic resistance properties

Staphylococcus aureus is an important opportunistic pathogen responsible for a variety of diseases. Indiscriminate prescription of antibiotics caused severe antibiotic resistance especially against commonly used drugs. The present investigation was carried out to study the distribution of Panton-Valentine Leukocidin gene, SCCmec types and antibiotic resistance properties of meticillin-resistant Staphylococcus aureus isolated from Iranian hospitals. A total of 132 clinical specimens were collected from two major Iranian hospitals. Samples were cultured and their positive results were subjected to several PCR methods. The patterns of antibiotic resistance were studied using the disk diffusion method. We found that 66 out of 132 samples (50%) were positive for Staphylococcus aureus. The most commonly infected samples were superficial and surgical wounds (66.12%). The incidence of mecA, tetK, ermA, ermC, tetM, aacA-D, linA, msrA, vatA, vatC and vatB antibiotic resistance genes were 80.30%, 34.84%, 30.30%, 25.75%, 24.24%, 19.69%, 7.57%, 7.57%, 6.06%, 3.03% and 1.51%, respectively. Totally, 40.90% of isolates harbored the Panton-Valentine Leukocidin gene. Of 53 mec positive strains, the distribution of SCCmec V, SCCmec III, SCCmec IVa, SCCmec IVc and SCCmec IVb were 28 (52.83%), 13 (24.52%), 6 (11.32%), 4 (7.54%) and 2 (3.77%), respectively. All isolates were resistant to penicillin, cephalothin, cefazoline and ceftriaxone. The high levels of Staphylococcus aureus resistance against commonly used antibiotics as well as high presence of SCCmec types of meticillin-resistant virulent strains of Staphylococcus aureus suggest that infections with these strains require more advanced hospital care with emerging demand for novel antibiotics.

Imipenem and meropenem activity against mecA-positive homogeneously and heterogeneously oxacillin-resistant and mecA-negative oxacillin-borderline-susceptible staphylococci

Microbroth dilution and disk-diffusion testing of imipenem and meropenem was performed at 35 and 30 degrees C against 61 phenotypic expression class 3,4 and 9 phenotypic expression class 1,2 oxacillin-resistant isolates of Staphylococcus aureus (ORSA), 51 oxacillin-borderline-susceptible isolates of S. aureus (BORSA), and 37 phenotypic expression class 3,4 and 9 phenotypic expression class 1,2 isolates of Staphylococcus epidermidis (ORSE). Imipenem MIC ranges at 35 degree C were 0.6 to > 64 micrograms/ml for class 3,4 ORSA, 0.03 to 0.25 micrograms/ml for class 1,2 ORSA, 0.015 to 0.12 micrograms/ml for BORSA, 0.03 to 64 micrograms/ml for class 3,4 ORSE, and 0.12 to 8 micrograms/ml for class 1,2 ORSE. Corresponding values for meropenem were 0.5 to 64 micrograms/ml, 0.12 to 4 micrograms/ml, 0.06 to 1 microgram/ml, 0.5 to 64 micrograms/ml, and 1 to 8 microgram/ml. MIC ranges at 30 degrees C did not differ by more than 1 log2 dilution from those at 35 degrees C. After 24 h incubation of disk-diffusion tests at 35 degrees C, 44% of class 3,4 and 100% of class 1,2 ORSA isolates were imipenem-susceptible; after an additional 24 h at 25 degrees C, 39 and 100% of these isolates, respectively, remained susceptible to imipenem. Similar values were obtained with 24 h incubation at 30 degrees C followed by 24 h at 25 degrees C. All BORSA isolates were susceptible to imipenem. Of the ORSE isolates, 22 and 78% of isolates in classes 3,4 and 1,2, respectively, were susceptible at 24 h with little change after an additional 24 h at 25 degrees C. Similar trends were observed with meropenem. In parallel disk-diffusion studies with oxacillin, false-susceptibility rates of 5% of class 3,4 and 44% class 1,2 ORSA isolates after 24 h of incubation at 35 degrees C were reduced to 3 and 0%, respectively, after an additional 24 h of incubation at 25 degrees C. Imipenem- and meropenem-resistant subpopulations of oxacillin-resistant staphylococci did not seem to be detected by altered susceptibility testing conditions.

Comparative in vitro activity of 11 beta-lactam antibiotics against 91 Staphylococcus intermedius strains isolated from staphylococcal dermatitis in dogs

A study was made of the susceptibility of 91 strains of Staphylococcus intermedius, isolated from staphylococcal dermatitis of dogs, to 11 beta-lactam antimicrobial agents. The activity of these antimicrobial agents against all strains was tested at pH 7.2 and 8.5. Finally, a comparative study was made of the susceptibility of these micro-organisms, dividing them into six groups: beta-lactamase positive and negative, methicillin resistant and susceptible and oxacillin resistant and susceptible. Only three of the 11 antimicrobial agents studied exhibited satisfactory behaviour at the two pH levels studied: mezlocillin, cefazolin and cefalotine. There was a good correlation at pH 8.5 between methicillin-resistant strains and those resistant to penicillin G, ampicillin, amoxycillin, cefamandole and cefuroxime. This correlation was not observed, however, between oxacillin-resistant strains and strains resistant to the other antimicrobial agents studied.

Changing causes of septicaemia in paediatric oncology patients: effect of imipenem use

One hundred and fifty-four episodes of septicaemia occurred in 78 patients on a Paediatric Oncology Unit over 2 years. Septicaemias with Gram-positive bacteria were more common than with Gram-negative organisms, coagulase-negative staphylococci (CNS) being the commonest pathogens. The mortality rate in patients with septicaemia was 1.9%. Azlocillin and gentamicin comprised the first-line of empirical antibiotic therapy for suspected infection for the first 10 months of the study; imipenem with cilastatin, as monotherapy, was used subsequently. More isolates of enterococci, and fewer isolates of Enterobacter, were seen after the introduction of imipenem. The use of imipenem was associated with an increased frequency of resistance to flucloxacillin in CNS. Such strains have been shown to contain sub-populations of cells that are resistant to imipenem. A clinical response was achieved in 82.9% of septicaemic episodes treated with imipenem, compared with 62.7% for azlocillin and gentamicin. However, imipenem as monotherapy may not be appropriate in central venous catheter related infections, owing to the frequent occurrence of imipenem-resistant organisms, CNS and Pseudomonas spp., in this situation.

Imipenem versus gentamicin combined with either cefuroxime or cephalothin as initial therapy for febrile neutropenic patients

A prospective randomized study was conducted to determine the efficacy of imipenem-cilastatin (hereafter referred to as imipenem) (500 mg four times daily) versus combination therapy for febrile neutropenic patients receiving either no prophylaxis or ciprofloxacin for prevention of infections. Combination therapy consisted of gentamicin (80 mg every 8 h) plus either cefuroxime (1,500 mg every 8 h) or cephalothin (1,000 mg every 4 h) for suspected catheter-related infections. Ninety-four neutropenic fever episodes in 87 patients were evaluable for efficacy. The overall clinical rate of response to imipenem was significantly higher than that to combination therapy (91 versus 74%; P = 0.05). The difference in efficacy was most pronounced in patients with microbiologically documented infections (89 versus 53%; P = 0.025), which were predominantly caused by gram-positive bacteria. Differences in susceptibility may have caused the better rate of response to imipenem. Two of 29 gram-positive bacteria were imipenem resistant, whereas 10 were resistant to cephalothin and cefuroxime and 12 were resistant to gentamicin. No causative gram-negative bacterium and 24 gram-positive bacteria were isolated in 61 fever episodes with ciprofloxacin prophylaxis (oral). In contrast, nine causative gram-negative and five gram-positive bacteria were isolated in 33 episodes without prophylaxis. The difference in distribution proved to be statistically significant for gram-negative (P = 0.0001) as well as gram-positive (P = 0.025) bacteria, indicating that ciprofloxacin effectively prevented the occurrence of gram-negative bacteria and may have contributed to the relatively large number of gram-positive bacteria isolated. Empirical initial therapy with imipenem may be a valuable alternative to combination therapy for neutropenic fever episodes.