S. aureus was isolated more frequently from the throat than from the anterior nares in two unrelated populations. Since the 259 patients were sampled upon admission to an orthopedic ward, their colonization status is probably unrelated within the group or to the 87 members of the staff in the same ward who were also sampled. The mean age of the patient group was much higher than the mean age of the staff group. Females were overrepresented in both groups, but no significant difference in isolation rates was observed between males and females in the patient group (data not shown). We therefore do not think our findings are related to sex or age, and we do not think the findings present a phenomenon in one single population.
Our results demonstrate that S. aureus
rarely was carried only in the anterior nares. If the anterior nares were colonized, the bacteria were, with few exceptions, also present in the throat. This could have some important implications. Since anterior naris carriage of S. aureus
is a well-documented risk factor for S. aureus
), prophylactic decolonization of carriers has been tried with different patient groups (12
). The spread of MRSA among patients and staff in health care institutions also calls for effective regimens for decolonization. A commonly used protocol includes topical treatment of the anterior nares with mupirocin, sometimes combined with application of disinfecting agents for the skin. For most patient groups, studies have not been able to show significant reduction of infection (12
). Some exceptions are hemodialysis patients (5
), peritoneal dialysis patients (1
), and general surgery patients (18
). Recolonization with the same strain of S. aureus
after the treatment period has been observed to occur (17
). The source of bacteria in these cases could be other carriage sites on the patient (for example, the throat) or people or items in the patient's environment. Repeated treatment has been shown to result in development of resistance to mupirocin (13
). During treatment of the anterior nares, low concentrations of mupirocin in the throat have been observed (24
). Furthermore, the effect of nasal mupirocin treatment on throat colonization has been questioned (8
). Taken together, this might mean that the standard protocol for decolonization of S. aureus
might create a situation favoring development of mupirocin resistance in the throats of treated individuals. Residual bacteria in the throat could also be one reason (of several) for recolonization after treatment. Kluytmans and Wertheim proposed that one way to increase efficacy of future treatment regimens could be to eliminate S. aureus
also from extranasal sites (12
). Considering our results, a treatment also targeting the throat might reduce problems with mupirocin resistance and recolonization and possibly lead to better efficacy in terms of infection reduction.
A large group was preferentially colonized only in the throat, in most cases with the same strain over several months and even years. Obviously, these individuals would not have been detected as carriers if only the anterior nares had been sampled. Nasal carriage seems to have a central role in S. aureus
epidemiology and pathogenesis of infection (11
), and the risks associated with throat carriage are largely unknown. Half of the preferential throat carriers were culture positive in the anterior nares at least once during the study period. Further studies are needed to determine what risk this group of S. aureus
carriers constitutes in terms of spread of the organism and infection. Meanwhile, it seems reasonable to use an enrichment broth (15
) and to include the throat when trying to identify MRSA and S. aureus
carriers in different situations. If only one site is to be sampled, the throat is probably a better choice than the anterior nares.
In this study, 39 of 67 individuals, or close to 60%, were characterized as persistent carriers. This is roughly twice or even three times as many as the number described earlier (27
). This discrepancy is probably due to the combination of a sensitive screening technique, the inclusion of the throat as a carriage site, and a different definition of persistent carriage (CI > 0.5). The change of definition is mainly due to the group of preferential throat carriers. Intervening negative samples reducing the CI for this group were probably due to false-negative results and not the fact that the individual had become a noncarrier. This conclusion seems reasonable since in most cases the same strain was recovered before and after negative sampling occasions. The observed lower CI for preferential throat carriers means that sampling of the throat yields fewer organisms than sampling of the anterior nares either because the sampling technique is suboptimal or, more probably, because there are small numbers of organisms present at the sampling site. A probable reason for the higher-than-expected rate of S. aureus
in the throat (compared to the rate in the anterior nares) is that we increased the sensitivity by using an enrichment broth. Usage of the same broth with methicillin increased the rate of MRSA isolation by 35% in a previous study (15
We observed almost 20% higher carrier rates of S. aureus
during the spring months of March and April than in the summer month of August. The higher rate was more pronounced for the throat. Since colonization and adherence of S. aureus
(and other bacteria) to the pharynx increase during viral infection (7
), an explanation might be that the peak of upper respiratory tract infections during spring in Sweden results in a greater number of individuals colonized. Alternatively, viral infections cause an increase in the number of bacteria in the throat, thereby increasing the chance of a positive culture. This seasonal variation is probably also the explanation for the observation of a more than 10% higher carriage rate among staff than among patients. The first samples from most of the staff were taken in March, but patients were sampled from March through September. To our knowledge, a seasonal variation of S. aureus
carriage has not been described before but deserves further attention.
In conclusion, the throat has to be considered as an important carriage site for S. aureus and should be included when screening for S. aureus, including MRSA. Our results also show that S. aureus carriage in the anterior nares in most cases indicates presence of the organism in the throat, although in lower numbers. This has to be taken into account when designing decolonization protocols.