Micro TA Review 1
April 4, 2013
• Tips & hints
• Memorization… this is the time!
• Flashcards, stories, whatever it takes.
• You really do need to know this.
• Pathogen  clinical presentation & vice versa
• Questions: know all the answers
• It goes fast – try to keep up as best you can!
• Resources
• Clinical Micro Made Ridiculously Simple
• Lippincott Micro Cards
• TA PPTs
• Firecracker
Bacterial Cell Walls
Bacterial Genetics
TRANSFORMATION:
Ability to take up naked DNA of RELATED bacteria from environment
(especially S. pneumo, H. influenzae, and Neisseria -- SHiN)
TRANSPOSITION:
Segments of DNA that can “jump” from one chromosomal location to
another via excision and reincorporation. Can also jump from chromosome to
plasmid and vice versa.
TRANSDUCTION:
Bacteriophage (virus that infects
bacteria) transfers bacterial DNA
from one bacteria to another.
Can be generalized (lytic phage)
or specialized (lysogenic phage).
Bacterial
Genetics
CONJUGATION:
“Bacterial Sex”– may involve
UNRELATED bacteria, requires
direct contact and transfer is
UNIDIRECTIONAL. Two options:
1. F+ x F-: F plasmid contains genes
for conjugation and it is
transferred via pilus (no transfer
of chromosomal genes).
2. Hfr x F-: F plasmid can become
incorporated into DNA -- now
called an Hfr cell and now may
transfer plasmid + some flanking
chromosomal genes.
Normal Bacterial Flora
• Skin:
– Normal: Staphylococcus epidermidis
– Transient: S. aureus
• Upper Respiratory tract
– Normal: Anaerobic cocci, Viridans Streptococci
– Transient: Neisseria spp., Streptococcus pneumoniae,
Haemophilus influenzae
• Gastrointestinal Tract
– Anaerobic GNR (Bacteroides), Enterobacteriaceae (E. coli),
Enterococci
• Genitourinary tract
– Normal: Lactobacillus
– Transient: GBS
Virulence Factors
• Ability to block arms of immune system (phagocytosis,
antibodies, complement) (Staph aureus- Protein A; Strep pyogenes- M protein)
• Slime and biofilms (Staph epi, Strep viridans)
• Toxins (endotoxin vs. exotoxin) (Endotoxin: gram negatives; Exotoxin:
Staph aureus, Strep pyogenes)
• Adhesion (Staph sapro)
• Flagella
• Ability to survive phagocytosis (Strep pyogenes, hyaluronic acid capsule)
• Ability to survive in hostile environments
Exotoxin v. Endotoxin
Superantigens
Definition:
• Superantigen toxins allow the non-specific binding of
MHC II with T cell receptors, resulting in polyclonal
T cell activation (versus other toxins, which are just
monoclonal activation so not as intense).
• Polyclonal T-cell population  cytokine storm 
multisystem disease and shock.
Examples:
• S. aureus: TSST-1
• S. pyogenes: Exotoxin A
Antibiotic Targets
Antibiotic Classes
Cell Wall Synthesis Inhibitors: Peptidoglycan Cross-Linking
Penicillins, Cephalosporins, Carbapenems (Beta-Lactams)
• Mechanism: bind to transpeptidases (aka PBP) → inhibit cross-linking of
peptidoglycan  no cell wall  death by osmolysis
• Resistance via beta lactamases, altered penicillin binding protein, altered porins.
Cell Wall Synthesis Inhibitors: Peptidoglycan Synthesis
Vancomycin
• Mechanism: binds D-ala D-ala portion of cell wall precursors  inhibits
peptidoglycan synthesis.
• Resistance via amino acid change from D-ala D-ala to D-ala D-lac; gramnegatives are intrinsically resistant because vanco can’t cross outer membrane.
Protein Synthesis Inhibitors
Macrolides, Tetracyclines, Aminoglycosides
Inhibitors of DNA Polymerase Activity
Fluoroquinolones, Metronidazole
Folic Acid Synthesis Inhibitors
Sulfonamides and Trimethoprim (TMP/SMX)
OVERVIEW!
Do not be overwhelmed.
this week’s focus:
gram+ cocci
Gram Positive Cocci
Also see the table in your lab manual!
Types of Hemolysis
Also: Staph aureus,
GBS
Also: Strep
viridans
Enterococci have
variable hemolysis
Staphylococcus aureus
IN THE LAB
• Gram + cocci in clusters
• Coagulase +, catalase +
• Beta-hemolytic
cluster of grapes
EPIDEMIOLOGY
• Nosocomial and community, all ages
CLINICAL
• Pyogenic skin infxn (impetigo, cellulitis, abscesses)
• Acute endocarditis (v. S. viridans subacute)
• Osteomyelitis
• Septic arthritis
• Toxin-related syndromes (see below)
scalded skin
impetigo
VIRULENCE FACTORS
• Protein A, biofilm, capsule, coagulase, catalase, hemolysins, penicillinase
• Toxins: TSST-1 (superantigen, causes Toxic Shock Syndrome); enterotoxin (preformed
superantigen, causes gastroenteritis); exfoliative toxin (causes scalded skin syndrome)
TREATMENT
• Most strains secrete penicillinase  MSSA, treat with nafcillin
• Some strains alter the PBP  MRSA, treat with vancomycin
Staphylococcus epidermidis
IN THE LAB
Gram + cocci in clusters
Coagulase - (vs S. aureus), catalase +, novobiocin sensitive (vs S. sapro)
Gamma-hemolytic
•
•
•
EPIDEMIOLOGY
Nl. flora of the skin; nosocomial & iatrogenic
•
CLINICAL
Foreign body or device infxns (think prosthetics, IV lines, Foley
catheters)
Common contaminant in blood cultures
•
•
VIRULENCE FACTORS
Biofilm: extracellular scaffold that mediates attachment to foreign
devices
•
TREATMENT
Vancomycin & remove the infectious source (i.e., remove the device)
•
Staphylococcus saprophyticus
IN THE LAB
Gram + cocci in clusters
Coagulase - (vs S. aureus), catalase +, novobiocin resistant (vs S. epi)
Gamma-hemolytic
•
•
•
EPIDEMIOLOGY
Sexually active women  2nd most common cause of UTI (1st is E. coli)
•
CLINICAL
UTI (burning, urgency, frequency)
•
VIRULENCE FACTORS
Specialized mucosal receptors
•
TREATMENT
Folic acid synthesis inhibitors (TMP/SMX)
•
Streptococcus pyogenes (GAS)
IN THE LAB
• Gram + cocci in chains
• Catalase – (vs Staphs), bacitracin sensitive (vs GBS)
• Beta-hemolytic
EPIDEMIOLOGY
• All ages
strep throat w/oropharyngeal petechiae
CLINICAL
• Suppurative (bc of host neutrophil response): pharyngitis (“strep throat”), cellulitis (inc.
erysipelas), impetigo, necrotizing fasciitis
• Non-suppurative (toxigenic): scarlet fever, Streptococcal Toxic Shock Syndrome
• Delayed (immunologic): rheumatic fever, poststreptococcal glomerulonephritis
VIRULENCE FACTORS
• M protein, superantigen, streptolysin S & O, streptokinase, SpeA, B, C, anti-C5a peptidase,
hyaluronidase (in capsule), T antigen, DNAase
• Diagnose recent infxn with ASO titers (antistreptolysin O)
TREATMENT
• Penicillin
Streptococcus
agalactiae
Streptococcus
agalactiae
(GBS)
IN THE LAB
• Gram + cocci in chains
• Catalase – (vs Staphs), bacitracin resistant (vs S. pyogenes)
• Beta-hemolytic
EPIDEMIOLOGY
• Neonates, peripartum women, diabetics
CLINICAL
• Neonatal sepsis, pneumonia, meningitis
• Maternal sepsis
• Less commonly: UTI, endocarditis
VIRULENCE FACTORS
• Not important
TREATMENT/PROPHYLAXIS
• Beta-lactams (penicillin)
Group B is Bad for
Babies!
Enterococcus
Enterococcus (E.
faecalis, E. faecium)
IN THE LAB
• Gram + cocci in pairs or chains
• Catalase – (vs Staphs)
• Variable hemolysis (alpha or gamma)
• Subdivided into Lancefield Group D and non-Lancefield Group D depending on carbs in
cell wall
EPIDEMIOLOGY
• Nl GI flora; nosocomial infxn (esp VRE)
CLINICAL
• UTI, subacute endocarditis, biliary tract infxn
• Peritonitis
• Nosociomal superinfxn  bacteremia
VIRULENCE FACTORS
• Dextran (glycocalyx) helps bind to heart valves
TREATMENT
• Inherent antibiotic resistance to cephalosporins
• Now have vancomycin resistance as well (VRE)  treat with daptomycin or ampicillin +
aminoglycoside
Strep viridans (S. mutans, S. sanguis)
IN THE LAB
• Gram + cocci in chains
• Catalase – (vs Staphs), optochin resistant (versus S. pneumo)
• Alpha-hemolytic
EPIDEMIOLOGY
• Nl oropharynx flora
CLINICAL
• S. mutans  dental caries (cavities)
• S. sanguis  subacute bacterial endocarditis (heart valve damage)
VIRULENCE FACTORS
• Dextran (glycocalyx) helps bind to heart valves
TREATMENT
• Penicillin (usually sensitive to lotsa antibiotics)
foliage growing on mitral
valve from oral cavity
Streptococcus pneumoniae
IN THE LAB
• Gram + lancet-shaped cocci in pairs (“diplococci”)
• Catalase – (vs Staphs), optochin sensitive (versus S. viridans)
• Alpha-hemolytic
EPIDEMIOLOGY
• See later lectures
CLINICAL
• Most common cause of MOPS: bacterial Meningitis in kids & elderly, Otitis media in kids,
community-acquired Pneumonia, Sinusitis.
• Also, sepsis.
VIRULENCE FACTORS
• Polysaccharide capsule  prevents phagocytosis.
• Diagnose with Quellung reaction (capsular “swellung”)
TREATMENT
• Penicillin
• Some resistance  high dose penicillin & cephalosporins
• Lotsa resistance  vancomycin