الفهرس 
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Abstract
Antimicrobial resistance has become such a growing global problem that, according to the Institute of Medicine, it may be a “paramount microbial threat of the twenty-first century.” (Weber and Courvalin, 2005).
Resistance has produced an increased burden of illness, longer hospitalizations, excess deaths, and greater health care costs (Reed et al., 2005 and Stratton, 2006)
There is considerable evidence of drug resistance reflected by strains of staphylococci and mostly it is thought to be mediated by penicillinase production (Owens and Watts, 1988).
Staphylococci are considered causative mastitis pathogens in both clinical and subclinical cases, and S. aureus is the most important pathogen among S. species related to sub-clinical intramammary infections in dairy cows leading to severe economic losses in industry worldwide (Godden et al., 2002).
A variety of antibiotics can be used against this organism, S. aureus mastitis has been found to respond poorly to antibiotic treatment. (Bakerma et al., 2006). The increased resistance of S. aureus isolates to several antimicrobial agents has been reported (Gentilini et al., 2002). What impacts the effectiveness of therapy since control methods of this organism from dairy herds requires treatment of infected mammary glands with effective antimicrobial agents (Kirkan et al., 2005).
The determination of antimicrobial susceptibility of clinical isolates is required not only for therapy but also for monitoring the spread of resistant strains throughout the populations. ß-lactam antibiotics are the most frequently used in intramammary infusion therapy. Bacterial resistance mechanisms to this class of antibiotics include production of ß-lactamase and low-affinity penicillin-binding protein 2a (PBP2a) determined by the presence of the chromosomal gene mecA. The latter, designated for methicillin resistance, precludes therapy with any of the currently available ß-lactam antibiotics, and may predict resistance to several classes of antibiotics (Rohrer et al., 2003 and Moon et al., 2007).
The isolation of S. aureus methicillin resistant (MRSA) from animals was first reported in 1972 following its detection in milk from mastitic cows. Several reports concerning MRSA infection in domestic animals, including dogs, cats, cattle, sheep, chickens, rabbits, and horses (Lee, 2003 and Goni et al., 2004). In recent years, the number of cases has shown an increasing trend (Rich and Roberts 2004 and Weese et al., 2005).
The bacterium is a colonizer of the skin and mucosae from which it can invade multiple organs. In livestock S. aureus is an important cause of mastitis, skin and soft tissue infections (SSTI) and to lesser extent infections of the locomotory system. Surgical site infections (SSI) in which S. aureus is isolated have been increasingly reported in small companion animals and horses (Safdar and Bradley, 2008).
The potential for animals to act as a source of S. aureus zoonotic infections of humans is exemplified by recent descriptions of human infections caused by pig-associated strains of MRSA (Rasigade, 2010 and Van der Mee-Marquet et al., 2011).
In addition, recent evidence implies that some S. aureus strains may have the capacity to colonise humans (Garcia-Alvarez et al., 2011) and that humans represent an important source of new pathogenic strains affecting livestock (Guinane et al., 2010 and Price et al., 2012).
MRSA among the most important pathogens in terms of increasing prevalence and impact. MRSA once caused predominantly nosocomial infections in immunocompromised patients. In recent years, however, infectious disease experts have noted an emergence of infections not associated with hospitalization, often referred to as community-acquired (CA) MRSA. The incidence has risen dramatically in the past decade (Johnson and Saravolatz, 2006; Bartlett, 2006)
In contrast to hospital (or healthcare) acquired MRSA (HA-MRSA), CAMRSA has a number of unique characteristics and may present an even greater threat to public health and a more significant challenge to clinicians.The determination of antimicrobial susceptibility of clinical isolates is required not only for therapy but also for monitoring the spread of resistant strains throughout the populations. ß-lactam antibiotics are the most frequently used in intramammary infusion therapy. Bacterial resistance mechanisms to this class of antibiotics include production of ß-lactamase and low-affinity penicillin-binding protein 2a (PBP2a) determined by the presence of the chromosomal gene mecA. The latter, designated for methicillin resistance, precludes therapy with any of the currently available ß-lactam antibiotics, and may predict resistance to The isolation of S. aureus methicillin resistant (MRSA) from animals was first reported in 1972 following its detection in milk from mastitic cows. Several reports concerning MRSA infection in domestic animals, including dogs, cats, cattle, sheep, chickens, rabbits, and horses (Lee, 2003 and Goni et al., 2004). In recent years, the number of cases has shown an increasing trend (Rich and Roberts 2004 and Weese et al., 2005).The bacterium is a colonizer of the skin and mucosae from which it can invade multiple organs. In livestock S. aureus is an important cause of mastitis, skin and soft tissue infections (SSTI) and to lesser extent infections of the locomotory system. Surgical site infections (SSI) in which S. aureus is isolated have been increasingly reported in small companion animals and horses (Safdar and Bradley, 2008).The potential for animals to act as a source of S. aureus zoonotic infections of humans is exemplified by recent descriptions of human infections caused by pig-associated strains of MRSA (Rasigade, 2010 and Van der Mee-Marquet et al., 2011). In addition, recent evidence implies that some S. aureus strains may have the capacity to colonise humans (Garcia-Alvarez et al., 2011) and that humans represent an important source of new pathogenic strains affecting livestock (Guinane et al., 2010 and Price et al., 2012).MRSA among the most important pathogens in terms of increasing prevalence and impact. MRSA once caused predominantly nosocomial infections in immunocompromised patients. In recent years, however, infectious disease experts have noted an emergence of infections not associated with hospitalization, often referred to as community-acquired (CA) MRSA. The incidence has risen dramatically in the past decade (Johnson and Saravolatz, 2006; Bartlett, 2006)In contrast to hospital (or healthcare) acquired MRSA (HA-MRSA), CAMRSA has a number of unique characteristics and may present an even greater threat to public health and a more significant challenge to clinicians.