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Department of Emergency Medicine, Erasmus Medical Center, Rotterdam, The Netherlands
Funny Bacteria Overview
(Unfortunately, Not Really Funny, Just Peculiar!)
First, to briefly review, bacteria come in three general groups: cocci, bacilli, and spirochetes. Bacteria usually have both a cell wall and a cell membrane.
Orientation to Bacteria
Spirochetes are strange, spiral bacteria. The thinnest bacteria are the spirochetes – often they cannot be seen with a regular microscope at all. (That is why “dark-field microscopy” is needed to make them show up.)
Some bacteria are variable in shape, and those are called pleiomorphic.
Mycoplasma is the smallest bacteria, and is about the same size as the largest viruses. Mycoplasma is also unusual because it lacks a cell wall – these bacteria only have a cell membrane. It is the smallest independently living organism we know of.
Gram–positive bacteria have a much thicker cell wall than gram negatives do. Gram negatives have a different composition for the outer portion of their membrane, and a space between the inner and outer layer where their resistance enzymes often live. Gram negatives also have endotoxin on their cell wall.
Mycobacteria are acid–fast. They will not gram stain, because their cell wall contains special lipids called mycolic acids.
Outside the cell wall, many bacteria have other specialized structures. One important structure shared by many bacteria is the glycocalyx coat (slime layer) which allows the bacteria to adhere to various surfaces. Other bacteria have capsules of polysaccharide which make it hard for phagocytes to eat them. This makes them much more virulent than the same bacteria without a capsule. Typical examples of encapsulated bacteria are Streptococcus pneumoniae and Neisseria species.
In addition to the enzymes contained in the periplasmic space between the cell wall layers of gram-negative bacteria, bacteria often have plasmids of double-stranded DNA in their cytoplasm. These plasmids replicate independently, and often provide resistance to various drugs or environmental situations.
Transposons, or genes that jump about on various bits of DNA in the cell, sometimes also provide coding for important resistance mechanisms. Transposons do not replicate independently – they are replicated whenever the DNA to which they are attached at that moment decides to replicate.
Peculiar Types of Bacteria
Mycobacteria (M. tuberculosis & leprae + M. marinum & scrofulaceum, MAI & Kansasii)
Mycobacteria are proper bacteria, but they are neither gram positive nor negative. They are, of course, acid-fast (due to the high lipid content of their cell wall – mycolic acids). The three types that cause disease in normal hosts are M. tuberculosis, M. bovis (comes from infected unpasteurized milk), and M. leprae (leprosy).
The “atypical” mycobacteria generally cause clinical disease in immunocompromised hosts. Most cause tuberculosis-like illness (organisms M. kansasii, M. avium-intracellulare). An important exception is M. avium intracellular (MAI), which causes cervical lymphadenitis in healthy kids, without immunocompromise.
Atypical mycobacteria that cause disease in normal hosts are M. marinum (swimming pool/fish tank granuloma – can be salt or freshwater) and M. scrofulaceum (one of the causes of “scrofula” – mycobacterial cervical adenitis – M. tuberculosis can also cause scrofula).
Actinomycetes (Actinomyces & Nocardia)
The two general groups of actinomycetes are Actinomyces and Nocardia species. These are true bacteria, but very strange because they form long, branching filaments that look like fungal hyphae. Nocardia is weakly acid-fast.
Actinomyces is part of the normal oral flora, and sometimes causes orofacial abscesses with the famous “sulfur granules.”
Nocardia is found in the environment, mainly in soil. It causes pulmonary infection in immunocompromised hosts, with the special features of abscesses and draining sinus tracts! In normal hosts, one can get pustular lesions at the site of bacterial entry.
Mycoplasma
Very small bacteria with a cell membrane only (no wall). The lack of a wall explains why beta-lactams and cephalosporins can’t touch them! Their shape is variable, due to their flexible cell membrane.
The cold agglutinin test is often positive (although not diagnostic) for mycoplasma infection. A positive cold agglutinin test means that the patient has IgM antibodies to type O blood, which will agglutinate the RBCs at cold temperatures (4 °C), but not at body temperature. Official diagnosis is via serology or PCR from nasopharyngeal swabs (now available at many centers).
Spirochetes (Treponema, Borrelia, Leptospira)
These guys look like very skinny corkscrews. They come in three varieties: Treponema (syphilis, and several tropical diseases such as yaws and pinta – all killed by PCN), Borrelia (Lyme disease and relapsing fever), and Leptospira (leptospirosis). Both Leptospira and Treponema species are too thin to be seen with light microscopy. All species are flexible and motile.
Treponema – Interestingly, there are nonpathogenic species that are part of the normal flora of human mucous membranes. Treponema cannot be cultured, but infection can be confirmed with a variety of tests (nonspecific ones are called “reagins,” whereas specific ones are “treponemal”).
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