MRSA isolates

Of the latter four agents, clindamycin has the most data supporting its use. However, the clinician must be aware of inducible clindamycin resistance. For CA-MRSA isolates determined to be resistant to erythromycin but sensitive to clindamycin, an additional laboratory analysis, known as the erythromycin-clindamycin D-zone test, is conducted to assess for inducible clindamycin resistance.15 Isolates for which the D-zone test indicates inducible resistance should not be treated with clindamycin. 

CCA-MRSA isolates resistant to erythromycin should be evaluated for inducible clindamycin resistance. 

CA-MRSA isolates resistant to erythromycin should be evaluated for inducible clindamycin resistance. cLimited clinical data exist for the treatment of MRSA infections. 

Application of this approach to the eight MRSA isolates of the present study led to the PFGE image displayed in Figure 9.4. It is quite apparent that the PFGE patterns of isolates 1 and 2 are nearly identical, as are those of isolates 3, 5, and 7. Likewise isolate 6 and the control are very similar. The PFGE pattern from isolate 4 identifies it as unique, though probably related to isolate 6 and the control. 

Figure 9.4 PFGE data for the eight MRSA isolates studied. C stands for control.

Figure 9.5 Representative MALDI-TOF mass spectra from each of the eight MRSA isolates.

TABLE 9.2 Composite Correlation Data for the Eight MRSA Isolates of Mass Spectra in Figure 9.5... 

The evolution of MRSA strains is not fully understood, but the same mechanisms of mutation and gene transfer that exist in other species provide a likely reason. The emergence of gentamicin resistance plasmids illustrates the evolutionary potential of translocatable elements [186], MRSA strains which are also resistant to this aminoglycoside antibiotic are referred to as MGRSA. This evolutionary progression is also responsible for the formation of the -lactamase-heavy metal resistant plasmids [250]. Some MRSA isolates are penicillin-resistant by virtue of the enzyme /J-lactamase, which pre-dates the use of /8-lactams [251], However, the spread of the phenotype has probably arisen as a result of selection caused by the widespread usage of methicillin in hospitals. 

In the last case the strain was identical to eight MRSA isolates obtained from hospitals in the New York City metropolitan area, and all eight isolates, but not control isolates, could be transformed in vitro to develop intermediate resistance to vancomycin. Both the presence of glyco-peptides and environmental factors, as demonstrated by increased resistance of S. aureus to antibiotics in the presence of prosthetic material in animals, can exert selective pressure to develop new resistance mechanisms (93-95). 

According to Elevens et al., the proportion of MRSA isolated in ICUs in US hospitals increased from 35.9% in 1992 to 64.4% in 2(X)3. However, a decrease in multidrug resistance among MRSA isolates was observed. The incidence of MRSA cenhal line-associated BSIs has decreased in recent years, presumably due to prevention efforts. ... 

The antibacterial effects of tea polyphenols extracted from Korean green tea (Camellia sinensis (L.) O. Kuntze (Theaceae)) against clinical isolates of methicil-lin-resistant Staphylococcus aureus (MRSA) were evaluated (Cho et al. 2008). Characterisation of the MIC of oxacillin for 30 Staphylococcus aureus strains isolated from patients treated with oxacillin identified 13 strains with an oxacillin MIC>4 pg/ml as MRSA (range 8-512 pg/ml), while 17 strains were methicillin-susceptible Staphylococcus aureus (MSSA range 0.25-0.5 pg/ml). The MICs of tea polyphenols ranged from 50 to 180 pg/ml for both the MSSA and the MRSA strains. The MICs of oxacillin for each of the 13 MRSA strains were reduced between 8- and 128-fold when these strains were coincubated with sub-MIC (<0.5 X MIC) levels of tea polyphenols, demonstrating that this combination was synergistic for all of the clinical MRSA isolates (Cho et al. 2008). 

It is important to determine (1) whether the isolate is methicillin-susceptible or methicillin-resistant and (2) whether the patient has a prosthetic valve. For patients with no prosthetic material, methicillin-susceptible staphylococci treatment should consist of a penicillinase-resistant penicillin (e.g., nafcillin or oxacillin) with or without gentamicin, and for methicillin-resistant strains, therapy should consist of vancomycin (see Table 71-4). Combination therapy with aminoglycosides, when used in these patients, typically is given only during the first 3 to 5 days of therapy. In the absence of prosthetic material, some treatment guidelines do not recommend combination therapy against MRSA. However, many clinicians may combine either gentamicin or rifampin with vancomycin if the patient is unresponsive to monotherapy. 

The second set of experiments featured very controlled growth conditions and relatively small (5 ml) cultures. All tubes were inoculated with a similar number of cells (based on OD600 measurements) and incubated for only 5 hours instead of 24. Their optical densities ranged from 1.33 (for the control) to 3.57 (for isolate 1). If MRSA is assumed to behave similar to E. coli, all eight MRSA cultures should have been in log phase when analyzed. Representative spectra from these eight cultures appear in Figure 9.6 and crosscorrelation results appear in Table 9.3. 

The majority of SSTIs are caused by gram-positive organisms and, less commonly, gram-negative bacteria present on the skin surface. Staphylococcus aureus and Streptococcus pyogenes account for the majority of SSTIs. Community-associated methicillin-resistant S. aureus (CA-MRSA) has recently emerged and it is often isolated in otherwise healthy patients. 

Local treatment of skin and soft tissue infections with antibiotic-containing ointments or solutions should not be used because it leads to allergic reactions and rapid development of bacterial resistance. In settings where MRSA or resistant Enterobacte-riaceae (like ESBL s gram negative bacteria with extended spectrum beta lactames) or Pseudomonas spp. occur, the empiric use of vancomycin and a carbapenem can be necessary. The risk of transmission of these organisms should be minimalised by hygienic and isolation measures. 

Researchers from Takeda have isolated the antibiotics TAN-1057 A-D 29, 30 from Flexibacter sp. [67]. These (S)-/7-Lys-derived dipeptides have promising activity against clinically problematic methicillin-resistant strains of Staphylococcus aureus (MRSA). The /7-amino acid moiety is essential for the biological activity of TAN-1057 A 29. The bisguanidine 29 seems to have a dual mode of action, involving... 

Infections limited to soft tissue will require between 7 and 10 days of intravenous therapy followed by an additional 14 days of oral therapy (total duration 2-4 weeks). If MRSA is isolated, intravenous vancomycin must not be switched to oral vancomycin which has negligible absorption from the gastrointestinal tract. Oral agents may be selected from rifampicin, tetracyclines, fusidic acid or trimethoprim depending on sensitivity data and a combination of two agents is recommended. Oral linezolid monotherapy is an effective alternative. 

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