Microbiology

Question 1

LO #1, 2,3: Identify characteristics of bacteria.

Answer 1

Infectious pathogens come in all shapes and sizes. Bacteria are prokaryotes typically measuring
0.3 – 2 µ in size that contain their own DNA and produce RNA.

Bacteria divide by binary fission, and can multiply very rapidly. Periods of rapid growth are called Log phase growth followed by a Stationary phase.

• Bacteria have rigid cell walls; components of the cell walls contribute to their Gram
staining properties:
- Gram-positive bacteria have a large amount of peptidoglycan in their outer
envelope; peptidoglycan absorbs a large amount of crystal violet and thus Gram-positive bacteria appear purple under the microscope.
- Gram-negative bacteria have only a small amount of peptidoglycan, making them
appear initially colorless after the decolorization step in Gram staining. However, the
counterstain safranin stains Gram-negative bacteria reddish-pink under the
microscope.
- Additional components of a bacterial cell wall include envelopes, slime layers, and pili.

• Bacteria are further characterized by their use of oxygen:
Classification Characteristics
Obligate aerobes-
Require oxygen
No fermentative pathways
Produce superoxide dismutase

Microaerophilic-
Require low but not full oxygen tension

Facultative anaerobes-
Respire aerobically until oxygen depleted, then will
ferment

Obligate anaerobes-
Are fermenters
Cannot use O2
Generally lack catalase
Lack superoxide dismutase

• Bacterial morphology also adds to the description of bacteria

• Cocci (round)
• Bacilli (rods)
• Spiral
• Staph (grape/cluster)
• Strep (chains)
• Bacterial capsules may contribute to their pathogenesis by helping the bacteria evade phagocytosis.
• Gram-positive bacteria contain lipoteichoic acid, which helps them attach to their host cells
• Many Gram-negative bacteria have pili, or protein-strands that help them attach to their host cells
• Bacteria also produce various enzymes which contribute to their pathogenesis: urease, catalase, and coagulase are all important such enzymes.

• Bacterial toxins are the most overt contributors to pathogenesis, and come in two types:
- Endotoxins, which are structural components of the bacterial cell wall.
Lipopolysaccharide (LPS) is the most described endotoxin and causes
disease by triggering pro-inflammatory cytokines, followed by an activation
cascade which ultimately causes shock.
- Exotoxins, which are produced and then secreted by bacteria. There are five
categories of exotoxins, based on their toxic effects:
o Protein synthesis inhibitors
o Neurotoxins
o Super-antigens
o cAMP inducers
o Cytolysins

Question 2

LO #1, 2, 3: Identify characteristics of viruses.

Answer 2

• Most common infectious agents in humans
• Simple and small, compared to other microbes
• Comprised of nucleic acid (DNA or RNA) with a protein covering and sometimes a lipid envelope
• Depend on host cell (enzymes and other replication factors) for survival; will hijack host cell machinery to replicate and propagate.
• Basic structure:
• Nucleic acid (DNA or RNA) protected by nucleocapsid (regular array of proteins)
• Nucleocapsid may be surrounded by lipid membrane = enveloped virus
• Viruses attach to host cells and insert their nucleic acid for translation, making multiple copies of themselves.
• Such infected cells may trigger an immune response during this process and/or after dying. This immune response is the major contributor to viral pathogenesis.
• Other factors included in pathogenesis are antigenic drifting and shifting, in which the virus evades an immune response by changing its antigenic appearance.

Question 3

LO #1: Identify characteristics of fungi.

Answer 3

• Fungi include Molds, Yeasts, and Mushrooms
• Fungi are eukaryotic.
• Cell walls contain complex carbohydrates and ergosterol; these serve as targets for both identification and antifungal therapy
• Morphology: Fungi exist in hyphal forms (consisting of filamentous units) which, can matted together form a Mycelium. Yeast are single-celled fungi, and sometimes grow to develop Pseudohyphae (such as Candida albicans)

• An important feature of many environmental fungi is that they can convert from yeast form to fungal (mold) form based on environmental temperature = Thermal
Dimorphism. Examples include Histoplasma, Blastomyces, and Coccidioides.

• Identification of fungi can be accomplished via special stains, culture, serology, and antigen detection.

• Special stains include KOH preparation, PAS (Periodic-acid-Schiff), and Silver staining. Culture requires specific, enriched media for many yeasts and fungi to grow.
However, some fungi (Candida albicans and Coccidioides) can grow on routine
blood agar.

• Serology and antigen detection methods all suffer from some variable differences in sensitivity and specific, largely due to technique of the technologist and also to the immunogenic properties of the antigen-antibody interactions. These techniques primarily are performed in a laboratory setting, but skin-reaction tests do exist for
some fungi. Skin-reaction tests depend on presence of intact delayed-type hypersensitivity to be sensitive.

Question 4

Identification of Microbes

Answer 4

Stains, Cultures, Serology, Antigen Detection, Microbial Statistics (Age Specific, Disease Specific, Resistance Epidemiology)