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Despite the promises of the household-products industry, almost every surface is covered in microorganisms almost all the time. Swab a countertop, your skin, or your dinner and you will find a little world—and that only covers the estimated 10% of bacteria that can be cultured! Obviously, trying to sterilize our patients (and our countertops) is fu-tile; we have to try to target the bad bacteria and let the rest happily crawl all over us. See Appendix 1 for an illustration of how “not clean” we are.
In the microbial world, bacteria lie toward the “less like us” end of the spectrum. They are prokaryotes, not eukaryotes like fungi, protozoa, and humans. Differences between bacterial and human cells in their anatomy, biochemistry, and the selectivity of antibiotic targets is what allows for the safe and efficacious use of antibiotics.
Differentiating bacteria that are responsible for infection from those just along for the ride can be difficult. Many bacteria that can cause human disease are also normal commensal flora, including Escherichia coli, Streptococcus pneumoniae, and Staphylococcus aureus. Thus, growth of one of these organisms from a culture is not necessarily synonymous with infection. Suspicion of infection is increased greatly if the organism grows from a
normally sterile site, such as the bloodstream or cerebrospinal fluid (CSF). Indicators of infection in nonsterile sites (such as sputum and wound cultures) are a high number of organisms, presence of inflammatory cells, and symptoms referable to the culture site (e.g., cough or dyspnea in a patient with a sputum culture growing S. pneumoniae, redness and pain in a patient with a skin culture growing S. aureus).
Definitive identification and susceptibility testing may take hours to months, depending on the organism and the methods used. Microscopic exam-
ination and staining may allow for rapid preliminary identification. For bacteria, the most important of these techniques is the Gram’s stain. Being able to interpret preliminary microbiology results will allow you to provide the most appropriate therapy to your patients as early as possible.
One of the most fundamental differences among types of bacteria is how they react to a Gram’s stain. Gram’s stain (crystal violet) is a substance that selectively stains the cell walls of Gram-positive bacteria but is easily washed away from Gram-negative bacteria. Why? In Gram-positive
bacteria, the outermost layer is a thick layer of peptidoglycan, a cellular substance that gives bacterial cells rigidity. In contrast, Gram-negative bacteria have an outer membrane of lipopolysaccharides that blocks the stain from adhering to the peptidoglycan within the cell. Gram-negatives also contain peptidoglycan, but in smaller amounts, and it is not the outermost layer of the cell. Both Gram-positive and Gram-negative organisms contain an inner cell membrane that separates the cell wall from the cytoplasm of the organism.