Staphylococci, methicillin-resistant MRSA

Very narrow spectrum, penicillinase-resistant drugs This subclass of penicillins includes methicillin (the prototype), nafcillin, and oxacillin. Their primary use is in the treatment of known or suspected staphylococcal infections. Methicillin-resistant staphylococci (MRSA) are resistant to other members of this subgroup and may be resistant to multiple antimicrobial drugs. 

Methicillin-resistent staphylococci are strains of staphylococci, which show resistance to a wide variety of antibiotics. They are named for their resistance to methicillin, a (3 -lactamase-resistant penicillin. Methicil-lin-resistante Staphylococcus aureus (MRSA) has become a serious problem particularly in hospitals. 

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. 

Nafcillin, oxacillin, cloxacillin, and dicloxacillin are more resistant to bacterial (3-lactamases than is penicillin G. Consequently, these antibiotics are effective against streptococci and most community-acquired penicillinase-producing staphylococci. Methicillin, which is no longer marketed in the United States, is another penicillinase-resistant antibiotic similar to nafcillin and oxacillin. For historical reasons, staphylococci resistant to oxacillin or nafcillin are labeled methicillin resistant. Many hospitals are reservoirs for MRSA and methi-cillin-resistant Staphylococcus epidermidis (MRSE). These nosocomial pathogens are resistant in vitro to all (3-lactam antibiotics. 

Vancomycin, daptomycin or linezolid (to cover staphylococci including methicillin-resistant Staphylococcus aureus (MRSA), streptococci and enterococci) plus metronidazole (to cover anaerobes) plus gentamicin, ciprofloxacin, ceftazidime or aztreonam (to cover Gram-negative organisms including Pseudomonas). 

Coagulase-negative staphylococci, such as Staphylococcus epidermidis, are the most common causes of catheter-related infections due to their ability to adhere to prosthetic material. Staphylococcus aureus, aerobic gramnegative bacilli, and Candida albicans are also common causes of catheter-related infections. Depending on local susceptibility patterns, methicillin-resistant S. aureus (MRSA) may represent up to 20% of all isolates. In contrast, upward of 80% of S. epidermidis are methicillin-resistant (MRSE). 

Mupirocin (Fig. 10.16C) is the main fermentation product obtained from Ps. fluorescens. Other pseudomonic acids (B, C and D) are also produced. Mupirocin is active predominantly against staphylococci and most streptococci, but Enterococcus faecalis and Gram-negative bacilli are resistant. There is also evidence of plasmid-mediated mupirocin resistance in some clinical isolates of Staph, aureus. Mupirocin is employed topically in eradicating nasal and skin carriage of staphylococci, including methicillin-resistant Staph, aureus (MRSA) colonization. 

Vancomycin is the drug of choice for these resistant organisms because most MRSAs and coagulase-negative staphylococci are susceptible to it (see Table 109-6). The presence or lack of a prosthetic heart valve in patients with a methicillin-resistant organism guides therapy and determines whether vancomycin should be used alone or, if a prosthetic valve is present, whether combination therapy is necessary (see Table 109-7). 

Antistaphylococcal penicillins Methicillin [meth i SILL in], naf-cillin [naf SILL in], oxacillin [ox a SILL in], cloxacillin [klox a SILL in], and dicloxacillin [dye klox a SILL in] are penicillinase-resistant penicillins. Their use is restricted to the treatment of infections caused by penicillinase-producing staphylococci. Because of its toxicity, methicillin is rarely used. Methicillin-resistarft strains of Staphylococcus aureus (MRSA), currently a serious source of nosocomial (hospital-acquired) infections, are usually susceptible to vancomycin, and rarely to ciprofloxacin or rifampin. 

Very narrow spectrum, beta-lactamase resistant nafcillin, methicillin, oxacillin ° Spectrum known or suspected staphylococci (not MRSA)... 

E) Staphylococci resistant to methicillin (MRSA) are usually sensitive to aminoglycosides... 

Staphylococcus epidennidis and S aureus are common pathogens associated with infection originating from intravenous lines. Most strains of these organisms responsible for hospital-acquired infections are resistant to methicillin. Vancomycin remains the drug of choice for both MRSA and MRSE. Quinupristin-dalfopristin is usually effective in the case of staphylococci resistant to vancomycin. 

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