CHAPTER 2
Medical Microbiology
2-2
Introduction
• Medical microbiology: study of collecting
and identifying pathogenic organisms and
suggesting medical management
– Numerous specialties (e.g., bacteriology,
virology, mycology, parasitology)
• This chapter focuses on bacteriology
– Insight into prevention of health
care–associated infections (HAIs) through
methods such as proper hand washing by
health care providers and patients
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History of Microbiology
• Antonie van Leeuwenhoek (1600–1670s)
– Developed the microscope; able to view the
motility of “wee beasties”; kept much of the
knowledge to himself
• Present day
– Numerous classifications of microbes
– Numerous classifications of antibiotics
– Numerous vaccines against disease
2-4
“Science” of Microbiology
• Science: an idea used to develop a theory
• Scientific inquiry: planned and deliberate
investigation
• Scientific theory: information based on
natural and physical phenomena; able to
be tested by unaffiliated researchers
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Bacteriology
• Binary fusion: process of bacteria
reproduction; cell copies DNA and
organelles, divides cytoplasm, splits in half
• Asexual reproduction: cells make identical
copies of themselves; no involvement of
another cell
2-6
Naming Bacteria
• Binomial nomenclature
– Genus: first (family) name; capitalized;
italicized; groups bacteria with similar
characteristics
– Species: second name; lowercase; italicized;
represents specific characteristics of a
particular group of bacteria
2-7
Bacterial Structure and Morphology
• Bacteria
─Anucleate
─Nucleoid (stores DNA)
─Glycocalyx (protection around cell wall:
capsular, or slime layer increasing resistance
to antibacterial agents)
─Some have no locomotion; others have cilia
or flagella
─Spores (produced by inactive bacteria; difficult
to kill)
(Continues)
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Bacterial Structure and Morphology
• Three basic shapes
─Cocci (round or spherical)
─Bacilli (rod-shaped)
─Spirella (spiral)
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Cocci
• Grouped by formation
─Mono (bacteria in single formation)
─Diplo (paired bacteria)
─Strepto (chain-like formation)
─Staphylo (clustered formation)
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Bacilli
• Rod-shaped
• May possess flagella
• Pairs (diplobacillus) and chains
(streptobacillus)
• E. coli (normal flora of intestinal tract; may
migrate to urinary tract causing UTI or to
blood stream causing bacteremia)
• May form spores
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Spirella
• Spirochetes
– Gram negative
– Spiral-shaped
– For example, syphilis, Lyme disease
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Guidelines for Specimen Collection
• Collect before administration of antibiotics
• Collect without contaminating specimen
• Obtain sufficient quantity of specimen from
site, not surrounding tissue
• Place specimen in appropriate container
or transport media
(Continues)
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Guidelines for Specimen Collection
• Label each specimen properly
• Transport to lab within allotted time
• Safe collection and handling lead to
– Accurate results
– Prevention of spread of disease to other
health care workers or patients
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Bacterial Staining
• Dyes/stains
– Lend color to bacteria allowing ease in
viewing/identification
– Provide information about classification and
arrangement of bacterial cells
– Simple stains (illustrate structure and
arrangement of bacterial cells)
– Differential stains (supply information on
composition of bacterial walls)
2-15
Gram Stain Technique
• Place specimen on slide
• Stain with crystal violet resulting in purple
hue of organisms
• Wash slide with water
• Flood with Gram’s iodine aiding adherence
of stain
• Apply alcohol decolorizing rinse
(Continues)
2-16
Gram Stain Technique
• Bacteria retaining purple color are gram
positive
• Counter-stain with red dye (safranin)
• Bacteria retaining red or pink are gram
negative (more difficult to treat due to
three layers with tough cell wall)
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Acid-Fast (Ziehl-Neelsen) Stain
• Used on genus Mycobacterium testing
causative agents of TB and leprosy
• Acid-fast organisms resist staining due to
presence of glycolipid and mycolic acid
layer
• Acid-fast bacteria retain carbolfuchsin
stain; appear red against blue background
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Growing and Testing Bacteria
• Culture and Sensitivity Test (C&S)
– After obtaining specimen, inoculate into
proper culture media containing nutrients
allowing for growth of microbes
– Place in incubator, maintaining stable
temperature and humidity level
– Test growth organisms with antibiotics to
determine best options for treatment
2-19
Culturing
• Medium (liquid broth, semisolid, or solid
agar) varies with specific bacteria
• Use Petri dish
– Clear (allowing visualization of growth)
– Specimen is spread/streaked on dish, pattern
dividing plate into four quadrants
– Place bottom up in incubator, preventing the
dripping of condensation onto colonies
(Continues)
2-20
Culturing
• Primary (first) culture: usually available
after 24–48 hours of incubation
• Mixed culture: identification of more than
one microorganism; must separate to yield
pure culture of only one species
• After isolation of microbes, examine to
identify pathogens
(Continues)
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Culturing
• Pathogens identified by
– Appearance
– Growth requirements
– Biochemical tests
– DNA
• These identifications may require several
days
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Susceptibility Testing
• After identification of microbes, determine
susceptibility to treatment with antibiotics
• Disk diffusion (Bauer-Kirby test)
– Provides qualitative information; shows zone
of inhibition
• Broth dilution
– Quantitative
– Identifies concentration of drug needed; MIC
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MIC and MBC Testing
• Minimal inhibitory concentration (MIC)
– Does not identify if organism is killed
• Minimum bactericidal concentration (MBC)
– Determines whether the organism is killed or
just inhibited in growth
• Testing with FDA-approved machines
– Expensive
– Unable to detect some resistant bacteria
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Empiric Antibiotic Therapy
• Treatment begins before C&S results
• Determination of prescribed antibiotic
– Type or location of suspected infection
– Usual bacteria noted with this type of infection
– Local bacterial resistance patterns
• After C&S results, antibiotic therapy may
need modification
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Bacterial Disease
• Bacteria cause disease by
– Destroying infected tissue
– Release of toxins into the body
• Endotoxins: found only in gram-negative bacteria
and Listeria (gram positive)
• Exotoxins: gram-positive and gram-negative
bacteria
(Continues)
2-26
Bacterial Disease
• Signs and symptoms
– High fever
– Swelling
– Pain
– Tachycardia (rapid pulse)
– Tachypnea (rapid breathing)
– Abnormal, foul-smelling drainage from site
• Treatment: antibiotics (kill prokaryotic
bacteria without harming eukaryotic cells)
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Antibiotic Classification
• Classified as
– Bacteriostatic agents
– Bactericidal agents
– Broad-spectrum
– Narrow-spectrum
– Aerobic
– Anaerobic
– Super-bacteria
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Summary
• Numerous specialties in the field of
microbiology
• Organisms have two names
– Genus
– Species
• Bacteria have three basic shapes
– Cocci
– Bacilli
– Spirella
(Continues)
2-29
Summary
• Extreme care needed when obtaining
specimens for culture
• Microbiology laboratory processes
specimen
– Stains the collected material
– Identifies bacteria
– Detects bacteria susceptibility
• Determination of antibiotic therapy
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