Methicillin-resistant Staphylococcus aureus (MRSA) is one of the most common pathogenic bacteria causing skin and soft tissue infections to life-threatening systemic infection such as toxic shock syndrome (TSS) and necrotizing pneumonia [1]. The mortality rate of severe, invasive MRSA infections is about 20% and it has been estimated that MRSA infections are the leading cause of death by an infectious pathogen in USA [2]. MRSA was first identified in the United Kingdom in 1961, only 2 years after introduction of methicillin [3]. From the 1980s, new strains of MRSA emerged which led to continuous pandemic infections of MRSA around the world. Now many countries report that MRSA strains account for approximately 25% - 50% of infectious Staphylococcus aureus in hospitals [4].

MRSA is due to acquisition of altered penicillin-binding protein PBP2a (PBP2’), encoded by the mecA gene, which is carried by amobile genetic element called staphylococcal cassette chromosome mec [5] [6]. Many MRSA clone have associated resistance to additional antibiotics, such as erythromycin, clindamycin, ciprofloxacin, and tetracycline [7]. Multidrug-resistant strains of Staphylococcus aureus are often only susceptible to vancomycin, an antibiotic with considerably lower efficiency compared to beta-lactams. Furthermore, vancomycin-resistant MRSA has been also reported [8].

The present study was conducted to find out the recent clinical characteristics of invasive MRSA isolates at general hospital in the central of Japan. Our result would contribute to more extensive surveillance study.

First of all, we confirmed that all Staphylococcus aureus isolates were resistant to oxacillin in this study. Thus, we defined those isolates as methicillin-resistant Staphylococcus aureus. Next, we compared the differences between invasive and noninvasive disease in this study. Invasive bacterial disease was defined as isolation of bacteria from a normally sterile body site. One hundred seventy-one MRSA were isolated among which 95 (55.6%) were from inpatient and 76 (44.4%) were from outpatient (Table 1). There was no significant difference of hospitalization between invasive and noninvasive disease. The age incidence among 0 - 1 years age group was 18 (10.5%) [invasive-0, noninvasive-18] (p = 0.01), among 1 - 10 years age group, 41 (24.0%) [invasive-9, noninvasive-32], among 11 - 40 years age group, 15 (8.8%) [invasive-8, noninvasive-7] (p = 0.02), in 41 - 60 years it was 5 (2.9%) [invasive-1, noninvasive-4] and in >60 years age group it was 92 (53.8%) [invasive-28, noninvasive-64] (Table 1). Most of the MRSA isolates were from the pediatrics (56/32.7%) [invasive-4, noninvasive-52 (p < 0.01) followed by respiratory medicine (25/14.6%) [invasive-6,noninvasive-19], general medicine (20/11.7%) [invasive-4, noninvasive-16] and dermatology (20/11.7%) [invasive-17, noninvasive-3] (p < 0.01) (Table 1). We also found the significant differences of department between invasive and non-invasive disease in surgery (p < 0.01). The results of antimicrobial susceptible patterns of MRSA isolates to various antibiotics tested in this study were shown in Table 2. Arbekacin, teicoplanin, and vancomycin were the most active antibiotics with 100% susceptible rates. Two antibiotics; erythromycin (p = 0.02) and gentamicin (p < 0.01) were more antimicrobial effective in invasive disease than in noninvasive disease significantly. There was no

significant difference of antimicrobial effect between invasive and noninvasive disease in other antibiotics. Surprisingly, our results showed that 123 (72%) cefazoline- and 138 (81%) imipenem-susceptible MRSA isolates by in vitro analysis.

In this study, we described the characteristics of invasive MRSA isolates from July 2014 to June 2015 at general hospital in the central region of Japan. With respect to hospitalized group, MRSA was isolated more from outpatients than inpatients. Our study showed the outpatient to inpatient ratio was about 1.25 times and there were no significant differences among gender. We clarified MRSA with age distribution. The present study reveals the prevalence of MRSA as seen in 0 - 1 years age group, it is 11%, increasing to 24% in 1 - 10 years age group, diminishing to 8.8% in 11 - 40 years age group and to 3% in 41%-60 years age group, and finally increasing to 53.8% in more than 60 years age group. Although young patients under 10 years frequently caused MRSA infection, the about half of MRSA were isolated from over 60 years age patients in our study. It is suggested to decrease immunity in the extremes of age groups.

In the analysis of clinical departments, we found that department where most patients with MRSA were detected was pediatrics. However, most MRSA were isolated from noninvasive disease in pediatrics. We presume that those isolates may colonize in patient. On the contrary, we found most MRSA caused invasive disease in surgery and dermatology. Staphylococcal skin disease frequently caused severe invasive disease such as TSS, Neonatal TSS-like exanthematous disease, and Staphylococcal scalded skin syndrome [1]. We may pay attention to infection-control management still more.

In the analysis of antimicrobial susceptibility, we did not find any vancomycin and teicoplanin-resistant MRSA in our study. A significant problem associated with MRSA is the subpopulation of MRSA with reduced vancomycin susceptibility in Japan [10]. However, the prevalence of MRSA with reduced vancomycin is low in Asian countries [11]. The recent trend of vancomycin-susceptible pattern of MRSA in Japan may be similar with that in Asian countries.

Furthermore arbekacin was effective antibacterial agent against all MRSA in our study. Arbekacin is classified as a kanamycin family aminoglycoside [12]. Arbekacin causes membrane damage and binds both to the 50s and 30S ribosomal subunits, resulting in codon misreading and inhibition of translation [13]. Arbekacin is not inactivated by aminoglycoside-inactivating enzymes [14]. Arbekacin shows the most potent antibacterial effect against clinically isolated MRSA strains among the aminoglycosides such as gentamicin, tobramycin, and amikacin [15], and the antibacterial effect of arbekacin is equivalent to that of vancomycin [15]. When MRSA is treated either with arbekacin or vancomycin with the same concentration, the post antibiotic effect of vancomycin was shorter compared with arbekacin [16]. Our results also suggested that arbekacin is potential first choice of anti-MRSA drug.

The 124 MRSA isolates were susceptible to clindamycin. Community-associated MRSA is known to be susceptible to clindamycin [17]. If the definition of this research was adapted to our results, about 72.5% of MRSA were considered as community associated MRSA. Community-associated MRSA contains SCC mec type IV or V [18]. Other researcher reported that SCC mec type IV strains are susceptible to imipenem [19]. If the definition of this research was adapted to our results, about 80.7% of MRSA were considered as SCC mec type IV strain. From these two definitions, at least 70% of MRSA may be community-associated MRSA. In contrast to most healthcare-associated MRSA, community-associated MRSA is susceptible to non-beta lactam antibiotics, and therefore, is not multidrug resistant [20]. We found that many MRSA isolates had not only imipenem but also cefazolin susceptible activity. Those MRSA may be community-associated MRSA. The susceptible pattern of MRSA in Japan may gradually change. Asian countries have shown over 50% rate of MRSA, which is the most important cause of healthcare-associated infections [11]. MRSA accounts for 25.5% of community-asso- ciated Staphylococcus infections and 67.4% of healthcare-associated infections in Asia [11]. The result of our study was opposite to this Asian studies. As this previous Asian studies did not include in Japan, the prevalence of community-associated Staphylococcus infections may be different from other Asian country. We suggest that community-associated MRSA spreads widely, in Japan, instead of healthcare-associated MRSA. Further molecular analysis is needed for the clarification of epidemiology of community-associated MRSA in Asian countries.

Incidence of MRSA infection is increasing worldwide and may lead to severe invasive infection by dissemination to other organs of the body if not treated adequately. The indiscriminate use of antibiotics has led to the emergence of multidrug resistance among commonly used antibiotics.

Our investigation aims to guide medical staffs on appropriate use of antibiotics. This aim is not only to reduce the morbidity and mortality in the patients but also to control the emergence and spread of resistance among MRSA. Continuous surveillance of the use of antibiotics helps in preserving the effectiveness of antibiotics. The results from our study strongly emphasize the need for continuous epidemiological monitoring of antibiotic resistant.

We thank Mr. Masashi Ishihara and Ms. Miwako Fujimura for special encouragement. This study was supported by a grant-in-aid for research from the Nagoya City University, Japan.

Masaaki Minami,Ryoko Sakakibara,Taichi Imura,Hideo Morita,Naoto Kanemaki,Michio Ohta, (2015) Clinical Characteristics of Invasive Methicillin-Resistant Staphylococcus aureus at General Hospital in the Central Region of Japan from July 2014 to June 2015. Journal of Biosciences and Medicines,03,9-14. doi: 10.4236/jbm.2015.39002