As discussed later in the chapter, B fragilis is capable of elaborating numerous virulence factors, which contribute to its pathogenicity and mortality in the host. B fragilis group species are the most common species recovered in some series of anaerobic bacteremia, and these organisms are associated with a very high mortality rate. Both B fragilis and B thetaiotaomicron are implicated in serious intrapelvic infections such as pelvic inflammatory disease and ovarian abscesses. These infections are often polymicrobial. In this study, the types of microorganisms were gram-positive bacilli, gram-positive coccus, and gram-negative bacilli. Bacteroides species are most often implicated in intra-abdominal infections, usually under circumstances of disruption of the intestinal wall as occurs in perforations related to surgery or trauma, acute appendicitis, and diverticulitis. Other commonly isolated members of the B fragilis group include Bacteroides ovatus, Bacteroides distasonis, Bacteroides vulgatus, and Bacteroides thetaiotaomicron. Strains are biochemically inactive compared with other members of the Enterobacteriaceae but will utilize glucose. They are facultative anaerobes and grow optimally at 37☌. Normal stools contain 10 11 B fragilis organisms per gram (compared with 10 8/g for facultative anaerobes). Members of the genus are small, motile, facultatively anaerobic Gram-negative rods. Often, bacteria that are facultative anaerobes are called “aerobes.”īacteroides species are normal inhabitants of the bowel and other sites. Streptococcus species and the Enterobacteriaceae (eg, Escherichia coli) are among the many facultative anaerobes that cause disease. Bacteroides and Clostridium species are examples of anaerobes.įacultative anaerobes: Bacteria that can grow either oxidatively, using oxygen as a terminal electron acceptor, or anaerobically, using fermentation reactions to obtain energy. Gram-negative bacteria have an outer membrane with lipopolysaccharides and a lipoprotein layer underlain by a thin peptidoglycan layer and the cytoplasmic. Mixed anaerobic infections can include both single anaerobic species or multiple anaerobic species with any number of nonanaerobic isolates. A functional definition of anaerobes is that they require reduced oxygen tension for growth and fail to grow on the surface of solid medium in 10% CO 2 in ambient air. Pathophysiology Symptoms and Signs Diagnosis Treatment Prevention Key Points Anaerobes can infect normal hosts and hosts with compromised resistance or damaged tissues. With mucosal damage due to disease, trauma or surgery, these organisms may invade tissues and set up infection. Some examples of facultatively anaerobic bacteria are Staphylococcus spp., Escherichia coli, Salmonella, Listeria spp., Shewanella oneidensis and Yersinia. Some Bacillus species and Mycobacterium tuberculosis are obligate aerobes (ie, they must have oxygen to survive).Īnaerobic bacteria: Bacteria that do not use oxygen for growth and metabolism but obtain their energy from fermentation reactions. Black-pigmented Gram-negative anaerobic rods are found on mucosal surfaces as indigenous flora. Aerobic bacteria: Bacteria that require oxygen as a terminal electron acceptor and will not grow under anaerobic conditions (ie, in the absence of O 2).
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