Anaerobes normal flora

Propionibacterium acnes (P. acnes), an anaerobic organism, is also found in the normal flora of the skin. This bacteria proliferates in the mixture of sebum and keratinocytes and can result in an inflammatory response producing a closed comedo or whitehead. More severe acne lesions such as pustules, papules, and nodules can also form with inflammatory acne and result in significant scarring if treated inadequately (Fig. 62-2). 

Antibiotics alter the normal colonic flora, leading to loss of colonization resistance, which is the ability of the normal flora to protect against overgrowth of pathogens, especially when the anaerobic flora are depleted [15], In CDAD, the altered colonization resistance can allow for the overgrowth of C. difficile in the colon. The bacteria produces two toxins which cause disease (toxin A, an enterotoxin, and toxin B, a cytotoxin). The toxins of C. difficile inactivate Rho proteins, which results in the loss of cytoskeletal integrity in enterocytes. Cellular damage results in fluid loss, exudation and diarrhea. The most severe form of C. difficile diarrhea is pseudomembranous colitis, which can cause severe colitis, toxic colon and rarely colon perforation and death. 

To understand the role of antibiotics, it is important to understand their effects on the fecal flora. The normal flora consists of a complex bacterial population with 400-500 distinct species of bacteria (table 2a). More than 99% of the total organisms are accounted for by non-sporeforming anaerobic rods [28] the four major species are Bacteroides, bifidobacteria, eubacteria and peptostrepto-cocci [29], Other common species are Escherichia coli, Streptococcus viridans, Streptococcus salivarius and lacto-bacilli. Mette et al. [30] clarified the prevalence of species in fecal flora by listing the four most common anaerobes Bacteroides spp., Eubacterium spp., Bifidobacterium spp. and anaerobic cocci) and three common aerobes E. coli spp., Enterococcus spp. and Lactobacillus spp.) (table 2b). 

Propionibacterium acnes, commonly isolated from the skin, is the most frequently found anaerobe. Factors and conditions such as blepharitis, dry eye syndrome, meibomian gland dysfunction, and contact lens use may influence the composition of the normal flora or cause disruption to normal epithelial microbial barriers, either of which can lead to disease in susceptible patients. Although immunocompromised individuals may harbor Candida albicans, fungi are considered opportunistic pathogens. Little evidence supports the existence of any indigenous fungi in the normal conjunctival flora. 

The mix of trapped keratinocytes and sebum provide an environment for the normally-occurring bacteria Propionibacterium acnes to flourish. Although P. acnes, a partial anaerobe, resides in the follicle as normal flora, it triggers immune responses such that titers of antibodies to P. acnes are higher in patients with severe acne than in non-acne control subjects. 

Aside from mild or severe allergic reaction, the most commonly experienced cephalosporin toxicities are mild and temporary nausea, vomiting, and diarrhea associated with disturbance of the normal flora. Rarely, a life-threatening pseudomembranous colitis diarrhea associated with the opportunistic and toxin-producing anaerobic pathogen, Clostridium difficile, can be experienced. Rare blood dyscrasias, which can even include aplastic anemia, also are seen. Certain structural types (details below) are associated with prolonged bleeding times and an antabuse-like acute alcohol intolerance. 

Microbes responsible for skin infection often arise from the normal skin flora which includes Staph, aureus. In addition Strep, pyogenes, Ps. aeruginosa and anaerobic bacteria are other recognized pathogens. Vimses also affect the skin and mucosal surfaces, either as a result of generalized infection or localized disease as in the case of herpes simplex. The latter is amenable to antiviral therapy in selected patients, although for the majority of patients, vims infections of the skin are self-limiting. 

There is also a segmental distribution of the types of bacteria. Strict anaerobic species are normally confined to the oral cavity and the colon, habitats they densely colonize and predominate [1-5] (fig. 1). Bacteria indigenous to the upper respiratory tract (URT flora) and anaerobic bacteria of oral origin are swallowed with saliva and recovered from the upper gut at densities below 105 CFU/ ml. Under physiological conditions, they are considered transitory rather than indigenous to the upper gut. Facultative anaerobic bacteria are usually confined to the distal small bowel and colon, but transient species entering the gut with nutrients are occasionally recovered from the healthy upper gut at low counts. 

Furthermore, similar glucose hydrogen breath tests in the elderly with and without omeprazole [76] and normal 14C-r/-xylose breath test in healthy old people with acquired gastric hypochlorhydria (pH >6) [32] counterindi-cate that H2 blockers induce colonization with strict anaerobes of intestinal types (colonic flora) in the upper gut. 

Infections caused by these injuries are most often caused by the normal oral flora, which includes both aerobic and anaerobic microorganisms. The most frequent aerobic organisms are Streptococcus spp., Staphylococcus spp., and Eikenella corrodens. The most common anaerobic organisms are Fusobacterium, Prevotella, Porphyromonas, and Peptostreptococcus spp. 

Major constituent of normal vaginal flora, facultative or anaerobic gram-positive rods increased numbers during pregnancy, during ovulation, at menopause, and during steroid treatment... 

Clostridium difficile is a commensal Gram-positive anaerobic bacterium of the human intestine, found in about 2-5% of the population. C. difficile is the most serious cause of antibiotic-associated diarrhoea and can lead to pseudomembranous colitis, a severe infection of the colon, often resulting from eradication of the normal gut flora by antibiotics. Discontinuation of causative antibiotic treatment is often curative. In more serious cases, oral administration of metronidazole or vancomycin is the treatment of choice. The bacterium produces several known toxins, including enterotoxin (toxin A) and cytotoxin (toxin B), both of which are responsible for the diarrhoea and inflammation seen in infected patients another toxin, binary toxin, has also been described. 

McNutt J,Allen SD,Wilson LA, et al.Anaerobic flora of the normal human conjunctival sac.Arch Ophthalmol 1978 96 1448. 

Acne vulgaris is the most common dermatologic disorder treated with either topical or systemic antibiotics. The anaerobe P. acnes is a component of normal skin flora that proliferates in the obstructed, lipid-rich lumen of the pdosebaceous unit, where tension is low. P. acnes generates free fatty acids that are irritants and may lead to microcomedo formation and result in the inflammatory lesions of acne. Suppression of cutaneous P. acnes with antibiotic therapy is correlated with clinical improvement. 

Common Sites of Invasion Anaerobes colonize the mouth, gastrointestinal tract and skin of all persons. Infections develop when anaerobes penetrate poorly oxygenated tissues (e.g., the diabetic foot) or tissues that are normally sterile (e.g., peritoneum). When broad spectrum antibiotics diminish normal bowel flora, C. difficile proliferates and releases a toxin that causes pseudomembranous colitis. 

Alterations of the intestinal microflora, due to frequent antibiotics administration accounts for the modification in qualitative fecal BA pattern, described for CF patients (increased percentage of primary BA)[6]. Moreover, a relationship between the anaerobic flora and BA loss in CF has been described[21] intravenous antibiotics therapy resulted in a reduction of anaerobic bacteria, decreased biotransformation of BA and improvement of BA absorption, with normalization of fecal BA losses. These results suggest that the reciprocal interactions between microbial flora and BA may also be of importance in determining BA malabsorption in CF. 

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