Intro to Clinical Microbiology Flashcards
List the major disease-producing microorganisms
Major: viruses bacteria fungi protozoa helminthes
Other:
prions
ectoparasites
List and describe the bacterial cellular morphology types—shapes and groupings
Shapes o Round (coccus): 0.5-1.25 μm diameter o Rod (bacillus): 0.5-1.0 μm diameter o Helical (spirochete): like rods but can be 10x longer
Groupings Cocci: • Singular • Pairs (diplococcic) • Chains (streptococci) • Groups of four (Sarcinae, tetrads) • Clusters (staphylococci) Bacilli: • Short coccobacilli • Long filamentous rods • Pointed ends (fusiform) • Curved (vibrio) • Pleomorphic (vary in size and shape) Spirochetes: vary in length and number of helical turns
Explain how the Gram stain is used in clinical microbiology
• Determined by cell wall structure of organism
Steps:
o Crystal violet stain (stains all bacteria on slide)
o Fixed with iodine
o Washed with alcohol or alcohol/acetone (decolorizes Gram-negative cell walls)
o Add safranin (red counterstain; able to be taken up by Gram-negative bacteria)
Results:
- Gram-positive = purple/blue (Thicker peptidoglycan (murein) cell wall blocks decolorization step)
- Gram-negative = red/pink
Describe the 4 phases of the bacterial growth curve and the relationship of the log phase to antimicrobial therapy and susceptibility testing
1) Lag phase:
o Cells are adapting/replenishing/undergoing size increases
o Little or no cell division
o No increase in cell number
2) Exponential (log) phase:
o State of maximal growth and constant rate of division
o Generation time = time for 1 cell to become 2 cells
o Optimal time to perform metabolic testing, antibiotic susceptibility/resistance testing
o Optimal time for antibiotic effectiveness
3) Stationary phase:
o Nutrients are exhausted, waste products accumulate
o Leads to decreased growth rate
o Rate of new cells forming = rate of dying cells
4) Death phase (or period of decline)
o Does not always occur
o Number of nonviable cells outnumber viable ones
Discuss settings where bacteria do not conform to laboratory growth pattern
o This lab pattern = growth in broth under optimal conditions
Exceptions:
• Biofilms: different physiology, metabolism, gene expression, antibiotic resistance than free-living (planktonic) bacteria
• Latent infections: bacteria appear to be in persistent stationary phase
Describe the temperature requirements of bacteria; define and discuss the clinical relevance of the following terms: psychrophile, mesophile, and thermophile
• Temperature: both optimal temperature for growth and range of temperatures allowing survival
o Mesophiles: 20-45 °C
• Most human pathogens
• Typical lab culture condition is 35-37 °C
o Psychrophiles: < 20 °C
• problems in refrigerated food or blood products
o Thermophiles: 45-60 °C
• problems in food processing (canning)
o Stenothermophiles: > 60 °C
Define and discuss the clinical relevance of the following terms: aerobe, anaerobe, facultative anaerobe, microaerophile, and aerotolerant.
Aerobes
o Oxygen required for growth
o Possess enzymes
o Microaerophiles: require reduced levels of oxygen (2-10%) because have low levels of enzymes
o Capnophiles: growth enhanced by increased level of CO2 (5-10%)
Anaerobes o Oxygen inhibits growth o Do not possess enzymes o Common in GI tract and mouth o Aerotolerant: can tolerate short oxygen exposure
Facultative
o Grow with or without oxygen
o Common in GI tract
o Most human pathogens
Describe the oxygen-detoxifying enzymes
Superoxide dismutase
• Superoxide radical (O2-) → H2O2 + O2
Catalase
• H2O2 → H2O + O2
Peroxidase
• Breaks down H2O2
Describe the components of bacterial genetic material
Chromosome
• Circular, dsDNA
• Replication precedes cell division
Plasmid
• Extrachromosomal circular genetic element with replication origin
• Usually 5- 100 genes
• Can be passed during cell division or transferred between bacteria by conjugation or transformation
• Usually not essential but confers selective advantage
• Ex. Antibiotic resistance, virulence factors, etc.
Transposon
• Genetic element contained on chromosome or plasmid
• Usually 1-10 genes (often including antibiotic resistance gene)
• Does not replicate independently
• Can move/jump from one site on DNA to another site on same DNA or to a different DNA molecule
List the mechanisms for changing a bacterial genome
1) Mutation
2) Genetic transfer
- transformation
- transduction
- conjugation
3) Genetic recombination
Describe the process of bacterial transformation
Mediated by free (naked) DNA from lysed organisms
• Uptake of free DNA
• Recombination with donor DNA and recipient DNA
Limited number of bacteria naturally competent for transformation
o Streptococcus pneumonia
o Haemophilus influenzae
o Neisseria
Describe the process of bacterial transduction
Mediated by bacterial virus (bacteriophage)
• Injects DNA or RNA into host bacterium
• Bacteria cell fills with new phages
• Lysis → release of new phages
2 types: 1) Generalized transduction • Mediated by lytic phage • Any portion of degraded bacterial DNA may be “mistakenly” packaged into assembling phage = transferred to another bacterium • A random error in packaging
2) Specialized transduction
• Mediated by temperate or lysogenic phage
• During lysogeny: temperate phage DNA integrates into bacterial chromosome at specific sites as a prophage (a latent viral infection that enables replication of the phage along with the chromosome)
• Certain conditions may cause excision of the integrated lysogenic prophage and initiation of a lytic cycle
• Imprecise excision → carries bacterial DNA sequences adjacent to site of integration = transferred to another bacterium
• Lysogenic conversion: if bacterial genes enable a nonvirulent organism to pick up virulence factors
Describe the process of bacterial conjugation
Mediated by transfer apparatus (cell-to-cell contact)
• Gram-negative bacteria: a sex pilus encoded by a fertility (F) plasmid
• Gram-positive bacteria: donor and recipient cells clump together via adhesions
• Transient cytoplasmic bridge forms through which plasmid or chromosomal DNA may transfer (donor → recipient)
• Can occur between related and unrelated bacteria
• Clinical importance: allows for passage of plasmids carrying virulence factors (including genes called R factors that encode for antibiotic resistance)
Discuss the clinical implications of lysogenic conversion.
• Occurs when phage infection and lysogenic prophage integration changes the phenotype of the host bacterium
Ex. Prophage may carry a gene encoding a virulence factor:
o Diphtheria toxin: only expressed by corynebacterium diphtheria strains lysogenized by the β prophage carrying the DT gene
o Cholera toxin: only expressed by vibrio cholera strains lysogenized by the CTX prophage carrying the CT gene
List and discuss the points to consider when doing a history and physical exam for an infectious disease
History
o Timing and nature of fevers
o Contact with others who are ill
o Predisposing factors (diabetes, immunosuppression, COPD, etc.)
o History of recent or recurrent infection
o Travel history
o Animal contacts
o Recent or current antimicrobial therapy
Exam
o Temperature
o Search for localized or generalized lymphadenopathy
o Skin (trauma, ulcers, line sites, rashes)
o Exam of each organ system
