DEPARTMENT OF MICROBIOLOGY,

FACULTY OF SCIENCE, UNIVERSITY OF PORT-HARCOURT,

PORT HARCOURT, RIVERS STATE, NIGERIA.

A study of factors affecting nasal carriage of Staphylococcus aureus among non-hospitalized persons at the University of Port-Harcourt, Port Harcourt, Nigeria, showed that carriage was influenced primarily by age (p=0.0344). Carriage was found to be 50% among primary school pupils, ages 5 to 11 years and 29.4% among university students and adult workers, ages 23 to 45 years. Other characteristics including gender, frequency of bathing, washing of hands, sickness, occurrence of catarrh or cold, nose bleeding and use of various medications did not influence carriage of S. aureus.

Keywords: Nasal carriage, Staphylococcus aureus, Port Harcourt, Nigeria

Staphylococcus aureus is a gram-positive bacterium endowed with virulent factors that make some strains highly pathogenic, causing infections ranging from boils to endocarditis. The reservoir of this pathogen is in the anterior nares of humans (carriers).

Interest in nasal carriage of Staphylococcus aureus dates back to several decades, because of concern in hospitals over nosocomial infections from carriers to patients (Moss et al., 1948; White, 1961; White and Smith, 1963; Yu et al., 1986) and for community acquired infections (Cole et al., 2001). Concern over carriage of antibiotic resistant strains led to studies on factors affecting carriage, its elimination and control (Reagan et al., 1991; Chenevert and King, 1997; Adams, 2002). These workers observed that duration of hospitalization was a risk factor for colonization by antibiotic resistant strains. Antiseptic wash was recommended for patients in intensive care units rather than isolation. Temporary de-colonization of nasal and hand carriage of methicillin resistant S. aureus (MRSA) was possible with intranasal application of mupirocin ointment. Adams (2002) reported that the strains of S. aureus present in both HIV-infected and non-HIV-infected individuals were similar to those found in healthy non-hospitalized carriers. Similarly, Yu et al., (1986) after a five-year study, reported that carriers tended to have chronic colonization with a single phage type. They found that S. aureus infections occurred more frequently in carriers and 93% of these infections were caused by the same phage type as that carried in the nares. Neither intravenous vancomycin nor topical bacitracin was effective in eliminating nasal carriage.

It has been suggested that S. aureus can be found in the anterior nares of practically all children and approximately 50 percent of adults (Treagan and Pulliam, 1982). Chin (2000) on the other hand, estimated nasal carriers to be 20% to 30% of the general population and stated that highest incidence of staphylococcal diseases is in areas where personal hygiene (especially the use of soap and water) is sub-optimal and people are crowded. Information on nasal carriage by Nigerian subjects is sparse.

This study was carried out to examine what factors influence S. aureus nasal carriage among non-hospitalized persons in the University of Port-Harcourt environment.

One hundred and two nasal swab samples were collected from 50 pupils (ages 5-11) of the University Demonstration Primary School (UDPS) and 52 workers and students (ages 23-45) at the University of Port Harcourt, Port Harcourt. Letters of introduction were given to the headmistress of the elementary school and the class teachers with an explanation of the study, which included questionnaire completion and taking of swab samples. Confidentiality of the results was also explained to all participants who were randomly selected from Elementary 1 and 6 classes and from various parts of the University. The questionnaires covered demography and health related characteristics of participants such as frequency of bathing, washing of hands, occurrence of catarrh/cold, sinusitis, nose bleeding, use of medications and nasal rub. The forms were completed before nasal swabs were collected.

The procedure followed was a modified version of that described by Treagan and Pulliam (1982). Sterile moist cotton swab was inserted about 1.5 cm into the subject’s nostril and gently rubbed over the inner surface. The swab was streaked over the entire surface of mannitol salt agar plate. The plates were incubatedfor 24hr. at 35 to 37⁰C. After incubation, type of culture, colony morphology, pigment production and mannitol fermentation were assessed. Colonies were purified on nutrient agar. Representative colonies were used for Gram staining and catalase test. Slide coagulase test was performed on colonies that resembled staphylococci macroscopically and microscopically using fresh citrated human sera obtained from the University of Port Harcourt Health Center. Data analysis was carried out using Epi-info version 6 software.

Observations on cultural characteristics of the nasal swab isolates showed that 96.0% of the swabs produced growth on mannitol salt agar, with 67.7% pure cultures and 32.3% mixed cultures. Following subculturing on nutrient agar, the predominant morphological types were gram-positive cocci (78.6%); 40% of isolates fermented mannitol and were coagulase positive. There were 40 (39.6%) S. aureus carriers, 25 were primary school pupils and 15 were students or workers.

Table 1 shows the distribution of health related characteristics of the participants by gender. Most participants regardless of gender had their bath twice a day (74.5%), 80.2% washed their hands before eating, 92.2% were not sick, 18.6% had catarrh or cold, 4.9% experienced nose bleeding and 13.7% were on various medications. There were no significant differences between males and females in the frequency of these characteristics.

Table 2 shows the influence of these various characteristic on S. aureus carriage. Only age had a significant effect on carriage with 50% carriage among the pupils and 29.4% among workers (p=0.0344).

The observations in this study of 39.6% S. aureus nasal carriage for the population studied, with 50% carriage among children and 29.4% for adults, are within the ranges estimated for carriage by Chin (2000), but the value for children appeared low by the estimate of Treagan and Pulliam (1982). These workers however did not specify the age brackets for their subjects. Methodology may affect results on carriage.

Table 2: Effect of characteristics of participants on nasal carriage of S. aureus

Treagan and Pulliam (1982) suggested using blood agar plates and mannitol salt agar plates for nasal swabs and tube or slide coagulase test for isolates. Cole et al., (2001) used tryptic soy agar with 5% sheep blood and the Staphyloslide latex rapid agglutination test, which identifies coagulase- positive and protein A-positive S. aureus. Yu et al., (1986) used phage typing for identification. A study could be done to compare the sensitivity of these procedures.

Factors responsible for higher carriage in children include various epithelial and mucous host factors such as surface glycoproteins and proteoglycans and immunologic response (Peacock et al., 2003). Cole et al., (2001) screened 230 donors of diverse ethnic and socioeconomic backgrounds and identified 62 (27%) S. aureus carriers. Nasal fluid from carriers was found defective in killing endogenous S. aureus and nasal carrier isolates of S. aureus but not a laboratory S. aureus strain. Also nasal fluid from carriers contained elevated concentrations of neutrophil- derived defensins, human neutrophil peptides 1 to 3, when compared with that from healthy donors and patients with acute rhinitis. Thus these workers concluded that nasal carriage of S. aureus takes hold in nasal fluid that is permissive for colonization and induces a local inflammatory response that fails to clear the colonizing bacteria. It is obvious that as found in this study, carriage is not influenced by medications, infections and bathing but by intrinsic immunologic factors. Antibiotic prophylaxis provides only temporary nasal de-colonization (Yu et al., 1986; Reagan et al., 1991).