Introduction

Staphylococcus aureus is one of the major human pathogens widely spread in the environment. It has persisted as an important hospital and community pathogen. They are responsible for more than 80 % of the supportive diseases encountered in medical practice (Kalsoom and Abdul, 2006). This organism has been found to be the most common bacterial agent recovered from blood stream infections, skin and soft tissue infections and hospital-acquired post-operative wound infections (Giacometi et al., 2000; Doern et al., 1999). It is also associated with a variety of clinical infections including septicemia, pneumonia, wound sepsis, septic arthritis, osteomyelitis with substantial rates of morbidity and mortality (Shopsin and Kreiswirth, 2001; Engemann et al., 2003; Cosgrove et al., 2003). About 30-50% of the human populations are Staphylococcus carriers (Bhatia and Zahoor, 2007). These bacteria can survive on dry surfaces increasing the chances of transmission. S. aureus infections can spread through contact with pus from an infected wound, skin to skin contact with infected person and contact with objects used by infected person.

Antimicrobial resistance has been noticed as one of the paramount microbial threats of the twenty first century (Smolinski et al., 2003). S. aureus has been a stumbling block for antimicrobial chemotherapy and the introduction of new classes of antimicrobial agents is usually followed by the emergence of resistant forms of pathogens (Alborzi et al., 2000). Methicillin-resistant Staphylococcus aureus (MRSA) represents a challenge for public health; as community associated infections appear to be on the increase in both adults and children in various regions and countries (Layton et al., 1995).

The present study reports the prevalence, antimicrobial resistance and the plasmid profile of S. aureus isolates from serum & urine samples of healthy individuals in Chennai, India.

Materials and Methods

Sample collection

Bacterial isolation

Haemolytic activity

Antibiotic susceptibility testing

MAR index

Plasmid DNA Isolation

Results

Among the 40 samples analyzed, 57.5 % (23/40) of the samples were positive for fermentation of Mannitol Salt Agar. All the isolates were identified as Gram positive cocci in clusters under microscopic examination. The isolates were positive for methyl red, Voges Proskauer, catalase, and coagulase test. The biochemical characteristics of the isolates from clinical samples are indicated in Table 1. All the isolates showed -haemolytic pattern on blood agar.

Figure 1

Table 1: Biochemical characteristics of the isolates from serum and urine samples collected among healthy individuals

Antibiotic resistance

The antibiotic resistance data of S. aureus isolates are shown in Table 2. The isolates were resistant to methicillin (100%), amoxicillin (91.3%), bacitracin (73.91) and novobiocin (86.95%). The isolates were sensitive to norfloxacin, gentamicin, kanamycin, tetracycline, vancomycin and nitrofurantoin. None of the isolates were susceptible to all the tested antibiotics.

Figure 2

Table 2: The antibiotic resistance data of isolates collected among healthy individuals

MAR Index

Plasmid DNA analysis

The plasmid DNA analysis of the isolates showed the presence of one plasmid in all the isolates. A 23 kb plasmid DNA was observed in all the isolates when compared with the Lambda DNA Hind III digest (Banglore genei, India) as shown in Fig. 1.

Figure 3

Fig.1. Plasmid profile of isolated from healthy carriers in Chennai city. Lane 1-3 (Serum isolates), 5-7 (Urine isolates) represents isolates harboring 23 kb plasmid. Lane 4 represents Lambda DNA digest. Lane 8 empty.

Discussion

This study is the first report on the prevalence of multiple antibiotic resistant S. aureus isolated from healthy individuals in Chennai city, India. Notably, all the samples were collected from healthy individuals with no distinct symptoms observed in them for any of the diseases caused by the pathogen of our interest. Among the samples tested, 55 % of serum samples and 60 % of urine samples were positive for S. aureus. It was also observed that higher prevalence of S. aureus was recorded in female urine samples (34.78%) (Fig.2). This data is in agreement with Onanuga et al. (2005) who reported 36 % incidence of S. aureus in the urine samples tested among the healthy women.

Figure 4

Fig.2. Prevalence of in serum and urine samples collected among healthy male & female respectively

Higher prevalence of S. aureus (57.5 %) in healthy individuals was observed in the present study, compared to results recorded from clinical samples by several workers. Anbumani et al. (2006) reported 38 % incidence of MRSA from the blood cultures in Chennai. The results of this study is also in contrary to the results recorded by Rajaduraipandi et al. (2006) who reported only 28.4 % and 14 % incidences of S. aureus respectively from the blood and urine samples collected from major southern districts of Tamil Nadu.

All the isolates were resistant against amoxicillin, methicillin, bacitracin and novobiocin. The isolates showed least antibiotic resistance against other antibiotics tested. Onanuga et al. (2005) reported 16 % and 89 % of the isolates from healthy woman were resistant to gentamicin and vancomycin respectively which is in contrary to the present study. Only one isolate from serum sample showed resistance to vancomycin. Rajaduraipandi et al. (2006) and Anbumani et al. (2006) reported none of the isolates were resistant to vancomycin. This variation in the drug resistance may be well related to the type of antimicrobial agents prescribed for treating various diseases in different geographical areas (Radu et al., 1997).

The MAR index value ranges between 0.18 and 0.63. All the isolates showed resistance to atleast 2 of 11 antibiotics tested. The incidence of S. aureus isolates based on MAR index was shown in figure 3.

Figure 5

Fig. 3. Percentage prevalence of in serum and urine samples based on MAR index

Several workers have reported the multiple antibiotic resistance patterns of S. aureus from clinical samples (Assadullah et al., 2003; Anupurba et al., 2003; Krishna et al., 2004). when low doses of antibiotics are used against bacteria, they inhibit the growth of susceptible bacteria, leaving the smaller number of already resistant bacteria to thrive and grow. These bacteria spread their resistance traits to other previously non-resistant cells then eventually affecting other cells (Craig, 1998).

The plasmid analysis of the isolates revealed the presence of 23kb plasmid in all the isolates. Resistance in MRSA is also related to a chromosomal mecA gene that specifies the production of an abnormal penicillin binding protein called PBP2a. Multiple drug resistance of S. aureus is due to several drug resistant genes in a single plasmid, each with its own resistance markers. Rahman et al. (2005) reported the presence of 23 kb plasmid in S. aureus strains isolated from skin lesion sample which is in agreement with the present study (figure 1). The results of Rahman et al. (2005) confirmed that the 23 kb plasmid isolated from S. aureus might be carrying the genes coding for multidrug resistance and the plasmids were transferable.

The results of the present study are in agreement to the postulation that healthy members are the highest reservoir of multidrug resistant bacteria (Lamikanra et al., 1996). However, this study involves only a small number of isolates. So, a multi centre study should be well planned and done to determine the prevalence of multidrug resistant S. aureus reservoirs in the society. It is also necessary to develop policies to restrict the use of antibiotics and also to establish monitoring systems for rapid identification of epidemics.

Acknowledgement