Page 168 - First Aid for the USMLE Step 1 2020, Thirtieth edition [MedicalBooksVN.com]_Neat

124 SEcTioN ii Microbiology ` microbiology—basic bacteriology Microbiology ` microbiology—basic bacteriology

` microbiology—basic bacteriology

Bacterial structures
strUctUre cHemical comPositioN FUNctioN
Appendages
Flagellum Proteins Motility
Pilus/fimbria Glycoprotein Mediate adherence of bacteria to cell surface;
sex pilus forms during conjugation
Specialized structures
Spore Keratin-like coat; dipicolinic acid; Gram ⊕ only
peptidoglycan, DNA Survival: resist dehydration, heat, chemicals
Cell envelope

Capsule Discrete layer usually made of polysaccharides Protects against phagocytosis
(and rarely proteins)
Slime (S) layer Loose network of polysaccharides Mediates adherence to surfaces, especially
foreign surfaces (eg, indwelling catheters)
Outer membrane Outer leaflet: contains endotoxin (LPS/LOS) Gram ⊝ only
Embedded proteins: porins and other outer Endotoxin: lipid A induces TNF and IL-1;
membrane proteins (OMPs) antigenic O polysaccharide component
Inner leaflet: phospholipids Most OMPs are antigenic
Porins: transport across outer membrane

Periplasm Space between cytoplasmic membrane Accumulates components exiting gram
and outer membrane in gram ⊝ bacterial ⊝ cells, including hydrolytic enzymes
(peptidoglycan in middle) (eg, β-lactamases)
Cell wall Peptidoglycan is a sugar backbone with peptide Net-like structure gives rigid support, protects
side chains cross-linked by transpeptidase against osmotic pressure damage
Cytoplasmic Phospholipid bilayer sac with embedded Site of oxidative and transport enzymes; PBPs
membrane proteins (eg, penicillin-binding proteins involved in cell wall synthesis
[PBPs]) and other enzymes Lipoteichoic acids induce TNF-α and IL-1
Lipoteichoic acids (gram positive) only extend
from membrane to exterior

Cell envelope

Unique to Common to both Unique to
gram gram
Flagellum
Lipoteichoic acid Pilus
Capsule

Endotoxin/LPS Outer
Porin membrane
Cell wall
Peptidoglycan Periplasmic space
(β-lactamase location)
Cytoplasmic
membrane
Gram Gram

FAS1_2019_03-Microbiology.indd 124 11/14/19 12:19 PM