Outbreak of methicillin-resistant Staphylococcus aureus infection associated with an outbreak of severe acute respiratory syndrome.

SIR—We read with great interest the recent article by Yap et al. [1]. The authors report a significant increase in the methicillin-resistant Staphylococcus aureus (MRSA) acquisition rate, with a very high rate of ventilator-associated pneumonia—caused mainly by MRSA—in patients with severe acute respiratory syndrome (SARS) in an intensive care unit (ICU) that admitted only patients with SARS. Paradoxically, this increase occurred after infection-control measures (including the wearing of gloves and gowns at all times) were upgraded because of the SARS outbreak and despite a low importation rate of MRSA into the ICU.

Yap et al. [1] provide 3 possible explanations for this observation. First, the practice of wearing gloves at all times may have led to poor compliance with hand hygiene, and the routine wearing of long-sleeved gowns, which were not changed between contact with patients, could also have contributed to cross-transmission of MRSA. Second, the heavy use of antimicrobials active against gram-negative organisms could have promoted the overgrowth of MRSA. Third, the SARS-associated coronavirus (SARS-CoV) may predispose patients to secondary infection with S. aureus.

We agree with these hypotheses, but we disagree with the conclusion that “cross-transmission of MRSA may be increased … if the [infection-control] measures included excessive use of gloves and gowns” [1, p. 515]. An alternative explanation for the significant increase in the rate of MRSA acquisition may be a viral-bacterial interaction between SARS-CoV and S. aureus, leading to an explosive airborne dispersal of S. aureus and a very efficient transmission of MRSA from colonized to noncolonized patients (the “cloud phenomenon”). This phenomenon was described by Eichenwald et al. [2], who showed that newborn infants who are nasally colonized with S. aureus produce significant airborne S. aureus dispersal and become highly contagious after infection with a respiratory virus. These babies caused explosive outbreaks of S. aureus infection in nurseries. Because they were literally surrounded by clouds of bacteria, they were called “cloud babies” [2]. We have recently shown that the same mechanism also occurs in certain adult nasal S. aureus carriers (“cloud adults”) [3-5]. Reports in the literature describe single health care workers nasally colonized with S. aureus who originated nosocomial S. aureus epidemics while experiencing a viral infection of the upper respiratory tract. This confirms that “cloud adults” can cause outbreaks [3, 6, 7]. Our data also indicate that clothing contaminated with S. aureus can amplify the dispersal of these bacteria into the air [4, 5], in agreement with previous observations [8, 9].

In conclusion, aerial dissemination of MRSA because of the “cloud phenomenon” may be the main reason for the described epidemic of MRSA infection. This may have occurred as a result of direct aerial dissemination or as a result of heavy contamination of the environment of colonized patients (including contamination of patient bedclothes or health care worker gowns). This, in combination with difficulties associated with frequently changing gloves and gowns, may have greatly facilitated MRSA cross-infection during the SARS outbreak.