Microbial overview ii
Jul 05, 2015
Whether you measure them by population (total number) or biomass (total weight), microbes are the most widespread forms of life present in almost any environment. There are more microbes in your digestive track (about 100 trillion) than you have cells in your body (about 10 trillion). The soil is also crowded with microbes. There is a rich nutrient source and organic waste.
Size: Varies with kinds of bacteria, and also related to their age and external environment.
Cocci: sphere, 1μm Bacilli: rods , 0.5-1 μm in width -3 μm in length Spiral bacteria: 1~3 μm in length and 0.3-0.6 μm in width
Bacteria multiply by binary fission, which is called asexual reproduction. If the conditions are favorable, they can divide themselves every 40 minutes, thus a single cell can produce as many as 236 cells in only 12 hours.
– rod/spherical: coccobacillus
– curved rod: vibrio
– curved rod that forms spirals: spirillum
– long helical cell: spirochete
Shapes of Bacterial CellsShapes of Bacterial Cells
Variety of other cell shapes:
When food is rich, microbial "behaviour" is very simple:
EAT, GROW, AND DIVIDEEAT, GROW, AND DIVIDE.
Bacteria multiply by:• binary fission,
sexual way: in this case, they exchange genetic information, located on circular DNA (Plasmids).
Generation time ： the time it takes for a population of bacteria to double in number
many common bacteria ：20~60 min
most common pathogens in the body ： 5-10 hours
Named by the Linnaean system: Genus species or Genus species .Example:
1.Escherichia (Genus) coli (species). • Named after Theodor Escherich, German
bacteriologist who discovered this organism in intestinal tract in 1885.He called it Bacterium coli, but it was
subsequently renamed in his respect.
2. Bacillus megaterium; a large rod shaped organism, member of the Genus Bacillus.
3. Streptococcus faecalis; fecal organism, member of the streptococci, a large genus, some members live in or on animal hosts.
o Name often reveals some characteristic feature.
o Note: Bacillus (one genus of bacteria, italicized) vs bacilli (general term for rods, not italicized)
• Normal microbiota prevent growth of pathogens.
• Normal microbiota produce growth factors such as folic acid and vitamin K.
• Resistance is the ability of the body to get rid off disease.
• Resistance factors include skin, stomach acid, and antimicrobial chemicals.
Microscopy Stained preparations
Gram-stain
Acid-fast stainZiehl-Neelsen
FluorescenceDirect, e.g. auramine
Immunofluorescence
• General purpose• Enriched• Selective• Differential
• General purpose:
• Supports growth of most non fastidious organisms
• Nutrient and TSA (Trypticase soy agar)
• Enriched
• Blood and chooclate, enriched with blood for growth of:
Fastidious organisms
Streptococcus pyogenes, Neisseria gonorrheae
• Selective:
• Favors the growth of one type of microorganisms and inhibits the growth of others
• Sabouraud (pH5.6),
• Selenite broth, containing: Sodium selenite
to isolate Salmonells typhi
• Antibiotics
• Differential Media:
• Distinguishes between different groups of bacteria on the basis of biochemical characteristics
• EMB, MacConkey, HE, XLD, Blood
Media: Providing Nutrients in the Laboratory
Media can be classified according to threeproperties:1. Physical state – liquid, semisolid and solid2. Chemical composition – synthetic
(chemically defined) and nonsynthetic (complex)
3. Functional type – general purpose, enriched, selective, differential, anaerobic, transport, assay, enumeration
Media: Providing Nutrients in the Laboratory
Most commonly used media:nutrient broth – liquid medium
containing beef extract and peptonenutrient agar – solid media containing
beef extract, peptone and agar
Physical States of Media
Liquid – broth; does not solidify
Semisolid – clot-like consistency; contains
solidifying agent (agar or gelatin)
Solid – firm surface for colony formation
contains solidifying agent e.g.
(Agar, Protein (egg), gelatin )
Most commonly used solidifying agent
Agar: A complex polysaccharide isolated from red algae. Solid at room temp, liquefies at boiling (100oC), does not resolidify until it cools to 42oC provides framework to hold moisture and nutrients not digestible for most microbes
GelatinEggs
• Isolation
Isolation Plate: Mixture of Escherichia coli and Staphylococcus epidermidis Grown on
TSA
Isolation techniques include:Streak plate techniquePour plate techniqueSpread plate technique
General purposeNon-selective
medium(All species
grow)Natural colour
Selective medium
(One species grows)
General purposeNon-differential
medium(All species have a
similar appearance)
Differential medium(All 3 species grow but show different
reactions)Colour as a media
reaction
Media: Selective & Differential
Obtaining Pure Cultures from an Isolation Plate
Obtaining Pure Cultures from an Isolation Plate
(Trypticase soy agar)
