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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.
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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
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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.
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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
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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.
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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)
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• 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.
26. • General purpose:
• Supports growth of most non
fastidious organisms
• Nutrient and TSA (Trypticase soy
agar)
27. • Enriched
• Blood and chooclate, enriched with
blood for growth of:
Fastidious organisms
Streptococcus pyogenes,
Neisseria gonorrheae
28. • 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
29. • Differential Media:
• Distinguishes between different
groups of bacteria on the basis of
biochemical characteristics
• EMB, MacConkey, HE, XLD, Blood
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Media: Providing Nutrients in the
Laboratory
Media can be classified according to three
properties:
1. Physical state – liquid, semisolid and solid
2. Chemical composition – synthetic
(chemically defined) and nonsynthetic
(complex)
3. Functional type – general purpose,
enriched, selective, differential, anaerobic,
transport, assay, enumeration
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Media: Providing Nutrients in the
Laboratory
Most commonly used media:
nutrient broth – liquid medium
containing beef extract and peptone
nutrient agar – solid media containing
beef extract, peptone and agar
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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 )
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Most commonly used solidifying agent
Agar: A complex polysaccharide isolated
from red algae. Solid at room temp,
liquefies at boiling (100o
C), does not
resolidify until it cools to 42o
C provides
framework to hold moisture and nutrients
not digestible for most microbes
Gelatin
Eggs
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General purpose
Non-selective
medium
(All species
grow)
Natural colour
Selective
medium
(One species
grows)
General purpose
Non-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
38. Obtaining Pure Cultures from an Isolation Plate
Obtaining Pure Cultures from an Isolation Plate
(Trypticase soy agar)
Bacteria multiply by:
binary fission, which is called asexual reproduction. If the conditions are favorable, they can divide themselves every 40 minutes.
However, they can also reproduce in a sexual way: in this case, they exchange genetic information, located on chains bicatenary of circular DNA .
Inoculation – introduction of a sample into a container of media to produce a culture of observable growth
Isolation –separating one species from another
Incubation – under conditions that allow growth
Inspection
Identification
