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2013-09-15 20:46:47

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  1. 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
  2. Plasmids
    • 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-
  3. Four ways bacteria can inter-transfer DNA.  This allows means for the dissemination (spread) of antibiotic resistance genes.
    • Conjugation
    • Transformation
    • Transduction
    • Transposition
  4. Conjugation
    • 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
  5. Transformation
    • uptake of free "naked" DNA from solution
    • Avery/McCleod experiment
  6. Transduction
    a bacterial virus (aka phage) that carries bacterial DNA into a new bacteria
  7. Transposition
    • 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.
  8. 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)