Microbiology:
Week Three

Chapter 4: Microbial Diversity Part Two

Bacteria: Characteristics

Bacteria are divided into 3 major phenotypic categories:

Those that are Gram-negative and have a cell wall
Those that are Gram-positive and have a cell wall
Those that lack a cell wall (Mycoplasma spp.)

•Characteristics of bacteria used in classification and identification include: cell morphology, staining reactions, motility, colony morphology, atmospheric requirements, nutritional requirements, biochemical and metabolic activities, enzymes that the organism produces, pathogenicity, and genetic composition.

Bacteria: Cell Morphology

There are 3 basic categories of bacteria, based on shape:

Cocci (round bacteria)
Bacilli (rod-shaped bacteria)
Curved and spiral-shaped bacteria.

Cocci:

Cocci may be seen singly or in pairs (diplococci), chains (streptococci), clusters (staphylococci), packets of 4 (tetrads), or packets of 8 (octads).
•The average coccus is about 1 µm in diameter.
•Some cocci have “coccus” in their name.

Bacilli:

Often referred to as rods; may be short or long, thick or thin, and pointed or with curved or blunt ends.
They may occur singly, in pairs (diplobacilli), in chains (streptobacilli), in long filaments, or branched.
An average sized bacillus is 1 x 3 µm
Extremely short bacilli are called coccobacilli.
Examples of medically important bacilli:

Escherichia, Klebsiella, and Proteus spp.

Pseudomonas, Haemophilus, and Bacillus spp.

Curved and Spiral-Shaped Bacteria:

Examples of curved bacteria

Vibrio spp.

Campylobacter spp.

Helicobacter spp.

Examples of spiral-shaped bacteria:

Treponema spp.

Borrelia spp.

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Bacteria: Motility

If a bacterium is able to “swim,” it is said to be motile.
Bacterial motility is most often associated with flagella; less often with axial filaments.
Most spiral-shaped bacteria and about 50% of bacilli are motile; cocci are generally nonmotile.

Bacteria: Atmospheric Requirements

Bacteria can be classified on the basis of their atmospheric requirements, including their relationship to O2 and CO2

With respect to O2, bacterial isolates can be classified as:

–Obligate aerobes

–Microaerophilic aerobes

–Facultative anaerobes

–Aerotolerant anaerobes

–Obligate anaerobes

Capnophilic organisms grow best in the presence of increased concentrations of CO2 (usually 5 to 10%)

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Bacteria: Nutritional Requirements

All bacteria need some form of the elements carbon, hydrogen, oxygen, sulfur, phosphorus, and nitrogen for growth.
Some bacteria require special elements (e.g., calcium, iron, or zinc).
Organisms with especially demanding nutritional requirements are said to be fastidious (“fussy”).
The nutritional needs of a particular organism are usually characteristic for that species and are sometimes important clues to its identity.

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Vocabulary words

Bacteria: Biochemical & Metabolic Activities

As bacteria grow, they produce many waste products and secretions, some of which are enzymes.
Pathogenic strains of many bacteria, like staphylococci and streptococci, can be tentatively identified by the enzymes they secrete.
In particular environments, some bacteria produce gases such as carbon dioxide or hydrogen sulfide.
To identify bacteria in the lab, they are inoculated into various substrates (i.e., carbohydrates and amino acids) to determine whether they possess the enzymes necessary to break down those substrates.

Bacteria: Pathogenicity

Many pathogens are able to cause disease because they possess capsules, pili, or endotoxins, or because they secrete exotoxins and exoenzymes that damage cells and tissues.

Frequently, pathogenicity is tested by injecting the organism into mice or cell cultures.
Examples of some common pathogenic bacteria:

Neisseria meningitidis

Salmonella typhi

Shigella spp.

Vibrio cholerae

Yersina pestis

Treponema pallidum

Microbiology: Week Three