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Bacteria that don't stain well with gram stain
- Acid fast bacteria (i.e. mycobacteria)--waxy coat of murein and lipids prevent gram staining
- Mycoplasmas (i.e. pneumoniae never have a cell wall
- L-forms--wall-less. Bacteria that have lost their cell walls but can re-grow them under right conditions
- spirochetes (syphilis). stained by fluorescene or silver staining
- small extrachromosomal autonomously replicating pieces of DNA that carry genes NOT essential for a cell's survival.
- They carry genes for antibiotic resistance and genes for enterotoxins and exotoxins
- Purpose: make up an extra pool of gene alleles, enlarging the effective gene pool of the population
- F plasmids (fertiility) has set of genes that makes conjugation possible. F+ contains these genes. Those that lack these genes are F-
Four ways bacteria can inter-transfer DNA. This allows means for the dissemination (spread) of antibiotic resistance genes.
- direct cell to cell transfer
- main way antibiotic resistance is transferred
- Needs F+ and F-
- works in gram -ve bacteria
- gram +ve have adhesins that cause aggregation
- uptake of free "naked" DNA from solution
- Avery/McCleod experiment
a bacterial virus (aka phage) that carries bacterial DNA into a new bacteria
- movement of bacterial DNA within a cell
- TN=transposable element
- Involves the movement of sequences throughout the genome with no requirement for sequence homology. The simplest of these elements are insertion sequences, which contain a transposase gene flanked by inverted repeats.
Methicillin Resistant Staph aureus = MRSA
- Community acquired MRSA is from the incorporation of a mobile genetic element called Staphylococcal cassette chromosome mec (SCCmec. mec stands for methicilin resistance gene)
- SCCmec is a transposable element that inserts in a site-specific location in the S. aureus genome. mecA gene produces penicillin binding proteins that bind to beta-lactam antibiotics at much lower efficiency that the intrinsic set of PBPs in S. aureus thus penicillin doesn't prevent cell wall synthesis.
- Larger cassettes encode multiple antibiotic resistance genes as well as other virulence factors
- There are different strains of MRSA differentiated by which SCmec cassette they harbor and which other virulence genes they possess. These different strains are categorized by MLST (multilocus sequence typing)