Microbiology II Flashcards
how are bacteria organized?
shape, arrangement, and staining
what shape are coccus (cocci)?
spherical
what shape are bacillus (bacilli)?
rod or cylinder
what shape are spiral bacteria (spirilla)?
twist like a spiral (motile)
what shape are pleomorphic bacteria?
range from cocci to rod
ex: cocobacilli
single bacterial arrangement
single spirilla
most spirillum and bacilli are single arrangement
pairs bacterial arrangement
diplococci
short or long chain bacterial arrangements
ex: streptococcus
clusters, branches, or group bacterial arrangements example
ex: staphylococcus
palisade bacterial arrangement example
ex: corynebacterium
endospores
non-reproductive structure that allows the bacteria to survive environmental stress
intracellular refractile bodies
can help ID the bacteria
might need a special stain to see them but can often see on a gram stain
what are endospores resistant to?
heat, desiccation, chemicals, radiation
which bacterium have central endospores?
Bacillus anthracis
which bacterium have subterminal endospores?
Clostridium chauvoei, Clostridium tetani
which bacterium have terminal endospores?
Clostridium botulinum, C. difficile
C. difficile causes supra-infections
supra-infection
an infection on top of another infection
how do stains help ID bacteria?
determines predominate organism in a mixed specimen, determines culture medium, determines appropriate anti-bacterial meds for sensitivity testing
gram stain
differential stain: differentiates gram positive from gram negative bacteria
gram positive stains purple, gram negative stains pink
gram positive bacteria in a gram stain
have thick complex cell wall which prevents leaching of the dye from the cytoplasm
gram negative bacteria in a gram stain
have thinner cell walls that contain lipids which can be freed allowing the cytoplasm to be decolorized
process of gram staining
- crystal violet (cells stain purple)
- iodine solution (cells remain purple)
- decolorizer (gram positive remain purple, gram negative become colorless)
- safranin (counterstain): gram negative become pink
common errors in gram staining
too thick, excessive heat fixing, under and over decolorization
under-decolorization
neutrophils that should stain pink are under-decolorized at decolorizer step and stained purple
over-decolorization
gives a false impression of gram negative rods being present
how can antibiotics change how organisms appear under the microscope?
can render organisms with a bizarre appearance
ex: can have gram variable or gram negative that normally stain positive because the cell wall is more permeable to the stain
what can make gram stain challenging?
Nocardia and actinomyces are weak gram positive and are small, frail, branching rods
we can further identify with acid fast stain if suspected
looking at a gram stain under the microscope
low power: can see signs of acute bacterial infection (segs) unless it’s a bacterial infection that destroys WBCs, see epithelial cells
oil: see bacteria
look at multiple areas of smear, make multiple smears
how to save a slide that stained badly
use xylol to remove the immersion oil
if under-decolorized: decolorize further and follow steps 3 and 4 (counterstaining)
if over-decolorized: do all 4 steps again
reporting gram stain results
use systematic, descriptive terminology
1. quantify entire smear
2. quantify individual bacteria
3. describe background (inflammatory: neutrophils, proteinaceous background: yeast)
fecal smear
normal psittacine stool consists of 95% gram positive rods and cocci and up to 5% gram negative rods with occasional yeast
KOH test for gram variable reaction
drop of potassium hydroxide is mixed with the sample
dissolves wall of gram-negative bacteria releasing DNA
gram negative: after 30secs-2 min of stirring a mucoid strand develops
gram positive: mixture will remain homogenous
simple stains used for microbiology
crystal violet: yeast, methylene blue: yeast, lactophenol blue: confirm and ID fungi
acid fast stain
penetrates resistant cells: these bacteria have a thick, waxy wall which makes them resistant and difficult to detect with gram stain
acid fast bacteria (bacteria that retain stain) stain red/pink
non-acid fast stain blue/green
used for Mycobacterium, Nocardia, and Crytosporidia
biochemical testing
media or chemical reagents that assist in identifying bacteria at a species level
basic approach: follow a predetermined step by step method of rule outs until you narrow the possibilities down to 1 species
usually done through flowcharts: different flowcharts for different species
catalase test
performed on gram positive cocci
catalase used is hydrogen peroxide which produces gas bubbles
catalase positive: staphylococcus
catalase negative: streptococcus, do CAMP test for S. agalactiae
coagulase test
performed on catalase-positive, gram positive cocci
coagulase positive: pathogenic Staph. aureus
coagulase negative: non-pathogenic Staph. epidermidis
CAMP test
performed on gram positive, catalase negative cocci
CAMP positive: Streptococcus agalactiae
CAMP negative: Streptococcus pyogenes
strep agalactiae is the only beta-hemolytic strep that hemolyzes RBCs on a CAMP test
oxidase test
performed on gram negative bacilli
oxidase positive: non-enterics, pseudomonas
oxidase negative: enterics (ex: E. coli, Salmonella, Shigella, Proteus)
oxidase reagent is added to a piece of filter paper with bacteria
a positive test will turn purple within 60sec, if it takes longer it’s negative
Indole test
identifies enterobacteria
2 methods: Kovac’s reagent broth and spot test
Kovac’s reagent broth: Indole positive will turn it red
spot test: Indole positive will turn it blue to blue-green
TSI test
triple sugar iron: lactose, sucrose, glucose, plus iron
distinguishes between enteric organisms: E. coli, Salmonella, Shigella, Proteus, Pseudomonas
multiple results that differ based on whether sugars are fermenting and whether gas is produced or not
bile esculin test
esculin: product from horse chestnut bark
selects organisms 1st on the basis of their ability to grow in a medium with 4% bile salts followed by selection based on their ability to hydrolyze esculin
identifies enterbacter organisms
what are examples of bacterium that test positive and negative on a bile esculin test?
positive: enterococcus faecalis
negative: escherichia coli
sulfur, indole, motility (SIM) test
tests for the reduction of sulfur to sulfide, the production of Indole, and motility
sulfur positive organism form black precipitate
Indole positive create a red reaction
motile organisms will make the entire culture turbid
what are some other tests we can use to test bacteria cultures?
commercially prepared kits:
enterotubes (enterics): gram negative
API: most commonly used for enterics, gram negative
immunology tests: ELISA, PCR
