Staphylococcus aureus: The new adventures of a legendary pathogen

COMMUNITY-ASSOCIATED MRSA IS INVADING THE HOSPITAL

In a new development, community-associated MRSA strains are now appearing in the hospital. This is not only because patients are bacteremic when they come in: patients in the hospital are getting nosocomial infections due to community-associated MRSA strains.

Seybold et al¹⁰ analyzed 116 cases of MRSA bloodstream infections in Atlanta, GA. In 9 (8%) of the cases the patient had not had any contact with the health care system within the past year, and these cases were classified as truly community-associated. Of the remaining 107 cases, 49 (42%) were nosocomial, and the USA 300 strain—the predominant community-associated MRSA strain—accounted for 10 (20%) of the nosocomial cases.

In the recent CDC study of invasive MRSA infections, Klevens et al⁴ reported that nearly a third of cases of bacteremia were due to community-associated MRSA, and these strains accounted for a greater proportion of cases of cellulitis and endocarditis than did health-care-associated strains.

In a study of hospital-associated MRSA, Maree et al¹¹ found that the percentage of cases in which the bacteria carried the SCCmec type IV marker had increased from less than 20% in 1999 to more than 50% in 2004.

Comment. Suffice it to say that we are surrounded by MRSA. Community-associated MRSA is here to stay. It is even invading our hospitals, and we need to consider this very carefully when choosing antimicrobial therapy.

NAGGING QUESTIONS ABOUT VANCOMYCIN

Case 2: Vancomycin-intermediate S aureus (VISA) bacteremia and endocarditis

In December 2006 we saw a very ill 60-year-old woman who was hospitalized with MRSA bacteremia, pacemaker endocarditis, and superior vena cava thrombosis. Although she was treated with vancomycin and rifampin, her condition worsened, she had a stroke, and she developed renal failure. In a difficult operation, the pacemaker was removed, but the bacteremia persisted. In early February 2007 she underwent another difficult operation in which the superior vena cava clot was debrided, a right atrial clot was removed, and her mitral valve was replaced. Less than 2 weeks later, and despite ongoing vancomycin and rifampin therapy, the MRSA bacteremia recurred.

During the approximately 6 weeks that the patient had been receiving these antibiotics, the minimal inhibitory concentration (MIC) of rifampin against the S aureus isolate increased from less than 1 μg/mL (susceptible) to 2 μg/mL (resistant). The MIC of vancomycin went from 2 μg/mL (susceptible) to 4 μg/mL (intermediately susceptible). Vancomycin and rifampin were discontinued, and daptomycin and gentamicin (Garamycin) therapy were started. (Her daptomycin MIC was 0.5 μg/mL). The patient’s condition stabilized, and she was discharged to a long-term nursing facility. She had no relapse of MRSA bacteremia, but she died in early April of that year.

Is vancomycin becoming less effective? Degrees of vancomycin resistance

Vancomycin has been our stalwart for treating MRSA infections for more than 40 years but it is not working as well as it used to, at least in certain situations.

Liu C, Chambers HF. Staphylococcus aureus with heterogeneous resistance to vancomycin: epidemiology, clinical significance, and critical assessment of diagnostic methods. Antimicrob Agents Chemother 2003; 47:3040–3045

VRSA (vancomycin-resistant S aureus) is rare. These fully resistant strains probably acquired a resistance mechanism (the vanA operon) from vancomycin-resistant enterococci. Infections tend to occur in patients simultaneously infected with both S aureus and vancomycin-resistant enterococci, giving the bacteria an opportunity to exchange genetic material.

VISA (vancomycin-intermediate S aureus) infections tend to occur in patients like the one described above who have had long-term vancomycin therapy. VISA strains appear to overproduce a matrix that captures vancomycin and keeps it from entering the cell. On electron microscopy, these bacteria have a very thick cell wall.¹³

Vancomycin tolerance is a state in which the bacteria are “stunned” or kept in check but not killed by vancomycin. That is manifested in the laboratory by a ratio of minimum bactericidal concentration to MIC greater than 32.

hVISA (heteroresistant VISA) is new and worrisome. These organisms have an overall MIC in the susceptible range, but within that population are individual isolates with an MIC that is much higher—in the intermediate or perhaps even in the resistant range.¹⁴

Reported rates of hVISA vary from less than 2% to as high as 76%, because the methods for detecting it are still very poorly standardized. The usual automated laboratory tests do not detect hVISA.

hVISA is probably clinically relevant, as evidence is emerging both in vitro and in vivo that the higher the MIC for vancomycin, the worse the clinical outcome.¹⁵ hVISA has been associated with failures of therapy in several situations, usually in cases of severe invasive or deep infection, endocarditis, and bacteremia with vertebral osteomyelitis where vancomycin concentrations at the site of infection may be suboptimal.^16–19 While most hVISA strains that have been described were resistant to methicillin, some were susceptible.

The E test is emerging as the standard test for hVISA. This test uses a plastic strip that contains gradually increasing concentrations of vancomycin along its length. Placed in the culture dish, the strip inhibits growth of the organism at its high-concentration end but not at its low-concentration end. If the sample contains hVISA, the cutoff is not well defined, with a few colonies growing at higher concentrations.