Staphylococcus

Staphylococcus aureus colonizes the anterior nares and other skin sites in 15% of community isolates. Of the tertiary care hospital isolates, more than 25% are resistant to all β-lactam antibiotics. The increasing prevalence of methicillin-resistant S aureus (MRSA) in tertiary referral hospitals appears to be related to the population of high-risk patients at such centers. Unfortunately, MRSA is now an increasingly common cause of serious infection in primary care settings as well.

Acute serious staphylococcal infections require immediate intravenous antibiotic therapy. A penicillinase-resistant penicillin or first-generation cephalosporin is normally used, pending the results of susceptibility tests. Since 1997, infections due to strains of S aureus with reduced susceptibility to vancomycin (glycopeptide-intermediate S aureus) have been identified, and their frequency is increasing throughout the world. Some reported cases have been successfully treated with various forms of combination therapy, and newer antibiotics including daptomycin, evernimicin, linezolid, ceftaroline, and quinupristin/dalfopristin.

Staphylococcus epidermidis is an almost universal inhabitant of the skin; it is present in up to 90% of skin cultures. It can cause infection when local defenses are compromised. Its characteristic adherence to prosthetic devices makes it the most common cause of prosthetic heart valve infections, and it is a common infectious organism of intravenous catheters and cerebrospinal fluid shunts.

Most isolates are resistant to methicillin and cephalosporins; therefore, the drug of choice is vancomycin, occasionally in combination with rifampin or gentamicin. Unfortunately, there have also been reports of vancomycin-resistant infections caused by coagulase-negative Staphylococcus. In addition to antibiotic therapy, management usually involves removal of the infected prosthetic device or vascular catheter.

Ophthalmic considerations The ongoing Antibiotic Resistance Monitoring in Ocular Microorganisms (ARMOR) study is a nationwide surveillance program of antibiotic resistance among ocular pathogens. The study has demonstrated the prevalence of methicillin resistance among staphylococcal isolates from ocular infections, as well as a high probability of concurrent resistance to fluoroquinolones, aminoglycosides, or macrolides. All staphylococcal isolates were susceptible to vancomycin, and overall ocular resistance did not increase during the 5-year study period.

• Asbell PA, Sanfilippo CM, Pillar CM, DeCory HH, Sahm DF, Morris TW. Antibiotic resistance among ocular pathogens in the United States: five-year results from the Antibiotic Resistance Monitoring in Ocular Microorganisms (ARMOR) Surveillance Study. JAMA Ophthalmol. 2015;133(12):1445–1454.

• Gould IM. Treatment of bacteraemia: meticillin-resistant Staphylococcus aureus (MRSA) to vancomycin-resistant S. aureus (VRSA). Int J Antimicrob Agents. 2013;42(Suppl):S17–21.

Excerpted from BCSC 2020-2021 series: Section 1 - Update on General Medicine. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.