CHAPTER ONE
INTRODUCTION
1.1 INTRODUCTION
In human medicine, the most important family of bacteria is Enterobacteriaceae, which includes genera and species that cause well-defined diseases, as well as nosocomial infections. The members of this family are Gram-negative, rod-shaped, non-spore-forming facultative anaerobes that ferment glucose and other sugars, reduce nitrate to nitrite, and produce catalase but seldom oxidase. Most Enterobacteriaceae are components of the gastrointestinal flora of humans and animals, although many are also widespread in the environment. Furthermore, these bacteria can cause many different infections, such as septicaemia, urinary tract infections, pneumonia, cholecystitis, cholangitis, peritonitis, wound infections, meningitis, and gastroenteritis, and they can give rise to sporadic infections or outbreaks (Donnenberg, 2009).
Salmonella and Shigella infections represent a major health problem worldwide, particularly in developing countries where they are recognized as the most frequent causes of morbidity and mortality (David and Frank, 2000, Mahbubur et al., 2007; Abdel et al., 2008). Life lost, together with the high costs to local public health care system, makes prevention and control a priority (Mahbubur et al., 2007; Yah et al., 2007a). The two pathogens have been associated with diarrhoea but the severity of the diarrhoea varies with the pathogens. Generally Shigella causes bloody diarrhoea while Salmonella induces non-bloody gastroenteritis. Antibiotic resistant Salmonella and Shigella are of global concern because they affect both developed and developing countries due to increased international travel (David and Frank, 2000, Dubois et al., 2007).These concerns have been further reinforced in recent years by the emergence of antimicrobial resistance among major groups of the enteric pathogens. The presence of antibiotic resistant bacteria from hospitalized patients throughout the world has been documented (Yah et al., 2007b).
Studies with Salmonella and Shigella are of particular relevance because these species can occupy multiple niches, including human and animal hosts (Martin et al., 1996, Levy, 1998; Khan, 2006). Reports have shown that the resistance of gastroenteric Salmonella and Shigella strains to antimicrobial agents is in large part due to the production of extended-spectrum ?lactamases (ESBLs) encoded on plasmids, as well as on the chromosome (David and Frank 2000). In Gram-negative pathogens, -lactamases remain the most important contributing factor to -lactam resistance, and their increasing prevalence, as well as their alarming evolution seem to be directly linked to the clinical use of novel sub-classes of -lactams (Medeiros, 1997).
Beta-lactamases are bacterial enzymes that inactivate -lactam antibiotics by hydrolysis, which result in ineffective compounds (Bush,2001). Beta-lactam antimicrobial agents such as Penicillins, Cephalosporins, monobactams and Carbapenems, are among the most common drugs for the treatment of bacterial infections and account for over 50% of global antibiotic consumption (Kotra, et al., 2007). Bacterial resistance to -lactam antibiotics has significantly increased in recent years and has been attributed to the spread of plasmid mediated ?lactamases. Some of these organisms have produced new forms of the older enzymes such as the extended-spectrum -lactamases (ESBLS) that can hydrolyze newer Cephalosporins and Aztreonam (Paterson and Bromo, 2005).
ESBLs are enzymes that mediate resistance to extended spectrum (third generation) Cephalosporins such as Ceftazidime, Cefotaxime and Ceftriaxone as well as Monobactams such as Aztreonam (NCCLS, 1999). These ESBLS have been found worldwide in many different genera of enterobacteriaceae (Bradford, 2001). More than 200 different natural ESBLs variants are known in an increasing variety of Gram-negative species (Bradford, 2001) with their distribution being far from uniform (Marchandin et al., 1999). With -lactams being the