Evolution of antibiotic resistance in Staphylococcus aureus: the role of the skin

Genetics of antimicrobial resistance in Staphylococcus aureus

Strains of Staphylococcus aureus that are resistant to multiple antimicrobial compounds, including most available classes of antibiotics and some antiseptics, are a major threat to patient care owing to their stubborn intransigence to chemotherapy and disinfection. This reality has stimulated extensive efforts to understand the genetic nature of the determinants encoding antimicrobial resistance, together with the mechanisms by which these determinants evolve over time and are spread within bacterial populations. Such studies have benefited from the application of molecular genetics and in recent years, the sequencing of over a dozen complete staphylococcal genomes. It is now evident that the evolution of multiresistance is driven by the acquisition of discrete preformed antimicrobial resistance genes that are exchanged between organisms via horizontal gene transfer. Nonetheless, chromosomal mutation is the catalyst of novel resistance determinants and is likely to have an enhanced influence with the ongoing introduction of synthetic antibiotics.

Interaction between methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-sensitive Staphylococcus aureus (MSSA)

In this paper we describe the in vitro interaction between three strains of methicillin-sensitive Staphylococcus aureus (MSSA) [NCTC 11561 and two strains derived from patients (PMSSA)] and endemic methicillin-resistant Staphylococcus aureus (EMRSA) 1, 3, 15 and 16. Mixed bacterial cultures of MSSA and EMRSA were incubated and subcultured after one, two and seven days. A proportion of MRSA in 50 randomly selected colonies was assessed. All strains of EMRSA (EMRSA 1, 3, 15 and 16) outgrew and virtually eradicated MSSA (NCTC 11561) after 24 h. The interaction between strains of PMSSA and the various strains of EMRSA was variable. PMSSA strain 1 was almost completely outgrown by EMRSA 1 and EMRSA 3 after seven days. Similarly there was a substantial increase of EMRSA 1 and 3 when tested against PMSSA strain 2. EMRSA 15 increased modestly against both strains, but EMRSA 16 failed to increase in proportion against either of the strains. We conclude that there is a complex interaction between various strains of EMRSA and MSSA. This interaction may have an important bearing on colonization of patients with MRSA.

Efficacy of a new cream formulation of mupirocin: comparison with oral and topical agents in experimental skin infections

A new cream formulation of mupirocin developed to improve patient compliance was compared with systemic and topical antibiotics commonly used to treat primary and secondary skin infections. A mouse surgical wound model infected with Staphylococcus aureus or Streptococcus pyogenes was used. Topical treatment was applied at 4 and 10 h postinfection or oral treatment at a clinically relevant dose was administered 4, 8, and 12 h postinfection; treatments were continued three times daily for a further 3 days. Mupirocin cream was significantly more effective than (P < 0.01; two of eight studies) or not significantly different from (six of eight studies) mupirocin ointment in reducing bacterial numbers. Mupirocin cream was similar in efficacy to oral flucloxacillin but significantly more effective (P < 0.001) than oral erythromycin. It was also similar in efficacy to cephalexin against S. pyogenes but superior against S. aureus (P < 0.01). Mupirocin cream had a similar efficacy to fusidic acid cream against S. aureus but was significantly superior against S. pyogenes (P < 0.01). A hamster impetigo model infected with S. aureus was also used. Topical or oral treatment was administered at 24 and 30 h postinfection (also 36 h postinfection for oral therapy) and then three times daily for a further 2 days. On day 5, mupirocin cream was significantly more effective than mupirocin ointment in one study (P < 0.01) and of similar efficacy in the other two studies. Mupirocin cream was not significantly different from fusidic acid cream or neomycin-bacitracin cream, but it was significantly superior (P < 0.01) to oral erythromycin and cephalexin. Mupirocin cream was as effective as, or superior to, oral and other topical agents commonly used for skin infections.

Resistance to fusidic acid

Resistance to fusidic acid is determined by a number of mechanisms. The best described are alterations in elongation factor G, which appear in natural mutants that are harboured at low rates in normal populations of staphylococci (10(6) to 10(8)). Altered drug permeability has also been described, and appears to be plasmid-borne. Binding by chloramphenicol acetyltransferase type I and efflux are other described mechanisms of resistance whose prevalence is unclear. A large number of studies have examined rates of fusidic acid resistance in staphylococci. Most show low levels of resistance. Studies where high levels of resistance have been seen are from areas of the hospital where cross infection is common. Rates of resistance have tended to be slightly higher in methicillin-resistant strains of Staphylococcus aureus. Studies on the evolution of resistance have shown no major trends to the emergence of resistance. In one case this is despite increasing use of both systemic and topical fusidic acid over more than 24 years. Selection for resistant variants during treatment was recognised early in vitro and in vivo. However, evidence suggests that it does not occur at high frequency in clinical practice. Nevertheless, accumulated experience is that fusidic acid in combination with other agents results in less resistance emergence.

The antibiotic susceptibility of Propionibacterium acnes and Staphylococcus epidermidis isolated from acne

We studied the susceptibility of antimicrobial agents to Propionibacterium acnes (P. acnes) and Staphylococcus epidermidis (S. epidermidis) isolated from acne patients. We measured the minimum inhibitory concentrations (MICs) of the following five drugs: roxithromycin (RXM), erythromycin (EM), clindamycin (CLDM), minocycline (MINO) and ofloxacin (OFLX), which are frequently used to treat acne and skin infections. We found many resistant strains of S. epidermidis and some resistant strains of P. acnes. There was a correlation between the resistance of S. epidermidis and the former therapy for acne, but no distinct correlation between the resistance of P. acnes and the former therapy for acne.

Comparative study on the antimicrobial effects of Hexomedine and Betadine on the human skin flora

Studies were carried out to detect the modifications, if any, on the peri-umbilical flora of six healthy volunteers after two or three daily applications of Hexomedine solution (HEX) and Betadine solution (PVI) repeated for five consecutive days. A standardized scrubbing method was used for bacterial sampling. Surviving bacteria were selected with both selective and non-selective media, and then identified by gas chromatographic fatty acid analysis. Both antiseptics were highly effective, showing both immediate and residual antimicrobial activities. The use of HEX led to a slight increase in Gram-positive cocci and a small decrease in coryneforms, but PVI produced a marked increase in Gram-positive cocci and a sharp decrease in coryneforms. The two antiseptics, however, caused no major alteration in the cutaneous microbial population. Indeed, neither the overgrowth of Gram-negative bacilli nor the emergence of resistant species was observed.

[In vitro bactericidal effects of ofloxacin on S. aureus]

Two Staphylococcus aureus (S. aureus) strains with differing susceptibility to penicillin G as expressed by the minimum inhibitory concentration were exposed in vitro over a period of 8 h to the continuously changing concentrations of ofloxacin achieved in human serum and cantharides blister fluid (CBF) after the single oral application of 600 mg. In both strains bacterial density was rapidly, but not totally reduced: a reduction by 99 percent took no longer than about 1 and 1.5 h resp. facing the serum level profile and about 1.5 h facing the tissue level profile. The maximum reduction of staphylococcal density in percent (kn) amounted to 0.0007 and 0.0034 and to 0.0038 and 0.0047 resp. Thus not only the serum level profiles but also the skin tissue level profiles obtainable in man proved highly effective in vitro against S. aureus irrespective of the bacterial suspectibility against penicillin G. Therefore oral ofloxacin should prove useful in staphylococcal diseases especially in so far as cutaneous tissue is involved.

Antimicrobial resistance of Staphylococcus aureus: genetic basis

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Topical mupirocin in the treatment of bacterial skin infections

Mupirocin is an investigational topical antibiotic used for treatment and prophylaxis of bacterial skin infections. Mupirocin differs from other antibiotics in its synthesis, structure, and mechanism of action. In vitro, mupirocin possesses antimicrobial activity against staphylococci, streptococci, Hemophilus influenzae, and Neisseria gonorrhoeae. Few studies comparing mupirocin to other topical antibiotics are available. Initial studies comparing mupirocin to inactive vehicle in the treatment of impetigo indicate an overall 92 percent pathogen eradication rate with active drug and 58 percent eradication rate with vehicle. Overall response to treatment of secondary skin infections was favorable in 91 percent of patients treated with mupirocin and 77 percent of those treated with vehicle. Although incidence is not greater than placebo, adverse effects have included pruritus, burning, dry skin, and erythema. Additional trials and clinical use should further help determine the role of mupirocin in the treatment of minor, primary, and secondary skin infections.

Development of antibiotic resistance by staphylococcus aureus in a single patient. Confirmation by phage typing, antibiograms, and plasmid analysis

A patient is described who had colonization and infection with strains of Staphylococcus aureus increasingly resistant to multiple antibiotics while receiving several courses of broad-spectrum antibiotics. S. aureus strains with different phage types, antibiograms, and plasmid profiles were isolated from different sites at the same time. After the patient was transferred to an intermediate-care service where he received only supportive care and no further antibiotics, culture samples from areas previously colonized or infected (sputum, anterior nares, perineal region, rectum) showed no S. aureus. This patient graphically demonstrates that antibiotics may create an environment allowing the overgrowth of resistant organisms and that withdrawal of this selection pressure may allow more sensitive organisms to recolonize.

Photochemotherapy of psoriasis: effects on bacteria and surface lipids in uninvolved skin

The effect of PUVA therapy for psoriasis on the microbial flora and on levels and composition of skin surface lipid has been studied in ten patients. Samples were obtained from clinically uninvolved skin from the back and the forearm. There was no consistent significant change in the viable counts of aerobic cocci or anaerobic coryneforms during theraapy. A transient rise in Staphylococcus aureus counts was thought to be due to the use of emollients rather than no effect of PUVA. Genetic changes in the microorganisms were not sought. There was an increase in the amount of total lipid (both casual levels and replacement sums) during therapy. An increase of free fatty acids at the expense of triglycerides did not reach statistical significance. The influence of PUVA on the skin flora seems to be less in vivo than anticipated from in vitro studies.

Acquisition of antibiotic resistance by Staphylococcus aureus in skin patients

Acquisition of resistance to neomycin, gentamicin, fusidic acid, or clindamycin has been observed in three strains of Staphylococcus aureus and data from three patients infected with these strains are presented in detail. Clindamycin resistance followed the expected pattern by appearing in a strain of Staph. aureus with dissociated resistance to erythromycin after treatment with erythromycin and clindamycin. Low-level resistance to fusidic acid appeared in two strains in the apparent absence of exposure to that antibiotic. Labile neomycin resistance was encountered in a previously sensitive strain after topical neomycin therapy. Gentamicin resistance appeared in all three strains after topical therapy. In all three strains, a labile resistance (presumably plasmid-mediated) occurred with minimum inhibitory concentrations (MICs) of 64-128 microgram/ml but in one strain a stable resistance with MIC over 3000 microgram/ml appeared.