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). 

Many areas of the human body are colonized with bacteria— this is known as normal flora. Infections often arise from one s own normal flora (also called an endogenous infection). Endogenous infection may occur when there are alterations in the normal flora (e.g., recent antimicrobial use may allow for overgrowth of other normal flora) or disruption of host defenses (e.g., a break or entry in the skin). Knowing what organisms reside where can help to guide empirical antimicrobial therapy (Fig. 66-1). In addition, it is beneficial to know what anatomic sites are normally sterile. These include the cerebrospinal fluid, blood, and urine. 

FIGURE 66-1. Normal flora and concentrations of bacteria (organisms per milliliter). 

Patients with a history of recent antimicrobial use may have altered normal flora or harbor resistant organisms. If a patient develops a new infection while on therapy, fails therapy, or has received antimicrobials recently, it is prudent to prescribe a different class of antimicrobial because resistance is likely. Previous hospitalization or health care utilization (e.g., residing in a nursing home, hemodialysis, and outpatient antimicrobial therapy) are risk factors for the acquisition of nosocomial pathogens, which are often resistant organisms. 

I Bite-wound infections generally are polymicrobial. On average, five different bacterial species can be isolated from an infected animal bite wound.3 Both the normal flora of the biter s mouth and that of the bite recipient s skin can be implicated. The bacteriology of the cat and dog mouth is quite similar. Pasteurella multocida, a gram-negative aerobe, is one of the predominant pathogens, isolated in up to 50% of dog and 75% of cat bites. S. viridans are the most frequently cultured bacteria from human bite wounds.3,43 Table 70-10 provides a comprehensive list of cat, dog, and human bite-wound pathogens. 

Normal flora Normal colonizing bacteria of a human host. 

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). 

The leading hypothesis for the development of chronic intestinal inflammation is that an abnormal immune response to normal flora might be crucial. This loss of tolerance might be due to a lack of regulatory mediators or cells, or a breakdown in barrier function which makes possible the access of inflammatory bacterial products to the local immune system, thereby overwhelming the normal regulation [3], These possibilities were supported by... 

Lactobacillus preparation is intended to replace colonic microflora. This supposedly restores intestinal functions and suppresses the growth of pathogenic microorganisms. However, a dairy product diet containing 200 to 400 g of lactose or dextrin is equally effective in recolonization of normal flora. 

Bacteria involved in SSI are either acquired from the patient s normal flora (endogenous) or from contamination during the surgical procedure (exogenous). 

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