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Classification of Microorganisms & Medium Formulation
Classification of Microorganisms:– Microbes can be classified into four major groups: – 1- Protozoa 2- Bacteria.
– 3- Fungi. 4- Viruses.
– 1- The Protozoa: These are unicellular organisms with protoplasm
– differentiated into nucleus and cytoplasm.
– Diameters in the range of 2-100 µm.
– The most important groups of medical protozoa are:
– A-Amoeba: Entamoeba species. Mode of Motility: pseudopodia.
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aB- Mastigophora: Mode of Motility: the Flagella.
Gastrointestinal flagellates: Giardia intestinalis
Urogenital flagellates: Trichomonas vaginalis
Tissue and blood flagellates: Trypanosoma, Leishmania .
Trypanosoma, and Leishmania .
a
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C- Ciliophora: motile by cilia.
Example: Balantidium coli.
D- Sporozoa: intracellular infection.
Example: Plasmodium that cause Malaria.
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2- The bacteria:Bacteria are unicellular prokaryotic microorganisms that multiply by binary
fission.
Bacteria can be classified according to morphology, arrangement, and staining reaction into the following groups:
1- Filamentous bacteria: Streptomyces: antibiotic producers. 2- True bacteria:
Cocci: Gram positive: Staphylococcus, Streptococcus.Gram negative: Neisseria.
Bacilli: Gram positive: Bacillus, Clostridum, Corynebacterium.Gram negative: Enterobacteriaceae, Brucella.
3- Spirochetes: Slender flexuous spiral bacteria.Borrelia, Treponema, Leptospira.
4- Mycoplasma: The Smallest bacteria that lack of a rigid cell wall. 5- Rickettsiae and Chlamydiae: intracellular parasites.
3- The Fungi:These are saprophytic or parasitic organisms possessing relatively rigid cell
walls.
Medical fungi can be divided into:1- Mould: Branching filaments; hyphae, mycelium. Usually 2 to 10 µm in width.
Example: Epidermophyton, Trichophyton,Microsporum, Aspergillus.
2- True Yeasts: these are ovoid or spherical cells that reproduce asexually by budding and sexually with formation of spores.
Example : Cryptococcus spp.
3- Dimorphic fungi:Produce a vegetative mycelium in artificialmedia, but are yeast like in infected lesions.
Example: Histoplasma.
4- Yeast- like fungi: Example: Candida ( Pseudomycelium).
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4- The viruses:
Viruses consist of DNA or RNA enclosed in a simple protein shell known as a capsid.
General properties of virusesThey are very small in size, from 20-300 ηm.
They contain one kind of nucleic acid (RNA or DNA) as their genome.
They are metabolically inertThey are obligate intracellular parasites.
They are only seen by electron microscope.Depend on the parasitized cell for survival and multiplication
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aa
Structure of bacterial cells:Size, Shape, and Arrangement of bacterial cells:
Morphology and arrangement of bacterial cells are criteria used for classification of bacteria into following groups:
1. Cocci (Singular: coccus).2. Rods (bacilli), (Singular: rod, bacillus).3. Vibrios (Singular: vibrio).4. Spirilla (singular :Spirillum)5. Spirochetes. (Singular: Spirochaete).
1. Cocci:These are round or oval bacteria measuring about 0.5-1.0 micrometer in
diameter. When they multiplying, cocci may form pairs, chains, or irregular groups.
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Cocci in pairs are called diplococci, for example, meningococci
and gonococci.
Cocci in chains are called streptococci, for example Streptococcus pyogens.
Cocci in irregular groups are called Staphytococci, for example, Staphylococcus aureus.
2. Rods (bacilli):
These are stick-like bacteria with rounded, square, or swollen ends. They measure 1-10 micrometer in length by 0.3-1.0 micrometer in width.
It may arranged in:
A- Chains, for example, Streptobacillus species.
B- Branching chains, for example, lactobacilli .
C- Mass together, for example, Mycobacterium leprae.
D- Remain attached at various angles resembling Chinese letters, for example, Corynebacterium diphtheria.
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3-Vibrios:
These are small slightly curved rods measuring 3-4 micrometer
in length by 0.5 micrometers in width.
Most vibrios are motile with a single flagellum at one end.
They show a rapid darting motility.
For example:
vibrio cholerae.
4-Spirochetes:
These are flexible, coiled, motile organism, 6-20 micrometer in length.
They progress by rapid body movements.
Spirochetes are divided into three main groups:
A- Treponemes. B- Borreliae. C- Leptospires.
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v Medium :
v Solid media
v Medium : a nutrient blend used to support microbial growth.
v There are three physical forms of media, broth, solid, and semisolid.
v Solid media are more versatile in their usage.
• Promote surface growth
• Used to isolate pure cultures
• Ideal for culture storage
• Helpful in the observation of biochemical reactions
• Used to make slants, deeps, and plates (named by medium)
Mohammed Laqqan
– A medium is sterilized (living organisms removed) before usage in the lab.
– Sterilization methods include; autoclaving, dry-heat, filtration, UV exposure and ethylene oxide.
– Culture: Is part of specimen grown in culture media.
– Culture Media: is a medium (liquid or solid) that contains nutrients to grow bacteria in vitro. Because sometimes we cannot identify with microscopical examination directly, and sometimes we do culture for antibiotic sensitivity testing.
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Properties of Media:Properties of Media:
–Note:
–Support the growth of the bacteria.–Should be nutritive (contains the required amount of
nutrients).–Suitable pH (neutral to slightly alkaline 7.3-7.4).–Suitable temperature, and suitable atmosphere.
(Bacteria grow at 370C)–Note: media are sterilized by autoclaving at 1210C and 2
atmosphere for 15-20 minutes. With the autoclave, all bacteria, fungi, viruses, and spores are destroyed. Some media can’t be sterilized by autoclaving because they contain eggs or carbohydrates .
Mohammed Laqqan
Forms of cultureForms of culturevSolid (agar):
vSemisolid agar
vSolid (agar): Is Broth plus agar (seaweed).Are prepared by adding a solidifying agent (agar 1.5 -3%).Prepared mainly in Petri dishes, but also in tubes and slopes. After growth the bacterial colonies are visible. e.g. blood agar, chocolate agar, MacConkey agar.
vSemisolid agar (soft agar): Contains small amounts of agar (0.5-0.7%). Used to check for motility and also used as a transport media for fragile organisms. Can have semisolid agar in Petri dishes or in tubes. In tubes it is usually slanted to increase surface area.
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–Liquid (Broth):
–Properties of agar:
–Liquid (Broth): Mostly used for biochemical tests (blood culture, Broth culture). Growth of bacteria is shown by turbidity in medium. e.g. Nutrient broth, Selenite F broth, alkaline peptone water.
–Properties of agar:• Some what like gelatin.
• It melts at 970C and solidifies at 370C.
• Comes as sold powder and then you add water to it.
Mohammed Laqqan
Types of Culture MediaTypes of Culture MediavSimple (basal, ordinary):vSimple (basal, ordinary): Culture Media: are media that contain the basic
nutrients (growth factors) that support the growth of bacteria without special nutrients, and they are used as basis of enriched media. E.g. Nutrient broth, nutrient agar, peptone water. They are for the growth of non-fastidious organisms like E. coli.
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–Enriched Culture Media:
–Selective Media:
• TSI• EMB
–Enriched Culture Media: are media that are enriched with: Whole blood e.g. blood agar. Lysed blood (heated to 80C) e.g. Chocolate agar.
–Selective Media: it is a media, which contains substances that prevent or slow the growth of microorganisms other than the bacteria for which the media is prepared for. For example
• TSI (triple sugar iron agar): slanted tube.• EMB (Eosin Methylene blue): enteric isolation media.
Mohammed Laqqan
vDifferential Media (indicators):vDifferential Media (indicators): Contains indicators, dyes, etc, to differentiate microorganisms. E.g. MacConkey agar, which contains neutral red (pH indicator) and is used to differentiate lactose fermenter and non-lactose fermenter. (E.g. E. coli and Salmonella).
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Common media used in Microbiology LaboratoryCommon media used in Microbiology Laboratory
–Chocolate Agar:
–MacConkey Agar:
–Chocolate Agar: blood agar prepared by heating blood to 95C until medium becomes brown or chocolate in color heating the blood releases broth X and V growth factors and also destroys the inhibitors of V factor. These factors are required for the growth of most species of Haemophilus and also Neisseria gonorrhoear.
–MacConkey Agar: an inhibitory and differential medium used to distinguish lactose- fermenting Gram-negative organism from non fermentation. Crystal violet, bile salts and neutral red are inhibitor agent. neutral red is the PH indicator.
Mohammed Laqqan
vMannitol Salt Agar ( MSA ):
vMueller Hinton Agar:
vMannitol Salt Agar ( MSA ): for selective isolation for coagulase positive, mannitol-fermenting staphylococcus.
• Mannitol fermentation by pathogenic staphylococci is indicated by a yellow halo surrounding the colonies.Sodium chloride is the inhibitor agent.Phenol red is the PH indicator.
vMueller Hinton Agar: rich medium that support the growth of most microorganism. It is commonly used for antibiotic susceptibility testing: disk diffusion antibiotic susceptibility; antibiotic serum level measurements; MBC determination.
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vSalmonella Shigella ( SS ) Agar :
vTriple Sugar Iron Agar (TSI):
vSalmonella Shigella ( SS ) Agar : isolation and differential medium for pathogenic Gram-negative bacilli in particular, Salmonella and Shigella. Inhibitor for Coliforms.
vTriple Sugar Iron Agar (TSI): this a key medium for use in beginning the identification of a Gram- negative bacilli of the enteric group. It contains glucose (0.1% ), Lactose (1%), sucrose(1%). And peptone (2%) as nutritional sources. Sodium Thiosulfate serves as the electron receptor for reduction of sulfur and production of H2S. Detects fermentation of sucrose, lactose, glucose, as well as production of hydrogen sulfide and /or gas . Phenol red is the PH indicator; ferric ammonium citrate is H2S indicator.
Mohammed Laqqan
