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Microbiology

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General Microbiology

Question
8 out of 97
 

Prokaryotes have:



A Well-defined nucleus
B Sterols

C Muramic acid
D All of the above

Ans. C Muramic acid

Prokaryotes (bacteria): single double stranded circular DNA molecule devoid of nuclear membrane, some may have plasmids, absence of membrane bound organelles and cytoplasmic streaming, presence of cell wall/ petidoglycan / muramic acid (exception Mycoplasma), absence of sterols (exception Mycoplasma).

Extra Edge

Gram-positive cell wall

a. ~ 80nm thick

b. Composed mostly of several layers of peptidoglycan made of murein monomers linked by tetrapeptide sidechains and pentapeptide crossbridges.

c. Also contains teichoic acid (cell wall TA’s, cell membrane TA’s/ lipoteichoic acids ); these play an important role in virulence for all Gram positive bacteria, are antigenic leading to formation of antiteichoic antibodies.

Gram negative cell wall

a. Thinner than gram positive bacterial cell wall and structurally more complex

b. Peptidoglycan layer single unit thick present in a periplasmic space present just exterior to the cell membrane. Chlamydia lack or have very low amounts of peptidoglycan in cell wall

c. Outside to peptidoglycan : outer membrane; phospholipid bilayer in which other large molecules embedded (outer membrane protein, porins )

d. Outer membrane anchored to peptidoglycan by lipophilic lipoprotein

e. Lipopolysaccharide (LPS) molecule unique to gram negative outer membrane

f. Complex glycolipid: lipid ‘A’, to which is attached core polysaccharide and a terminal series of repeat plysaccharide unit (O or the somatic antigen which is the major surface antigen). The O antigen

is the outermost part of the LPS and the most variable in comparison to the lipid A and core polysaccharide.

g. Firmly bound to the cell surface, released only when cells are lysed

h. Endotoxic acitivity: lipid ‘A’

i. Periplasmic space : b/w inner and outer membrane; contains peptidoglycan layer

Pathogenicity Islands

a. Large groups of genes that are associated with pathogenicity and are located on the bacterial chromosome are termed pathogenicity islands (PAIs).

b. They are large organized groups of genes, usually 10 to 200 kb in size.

c. The major properties of PAIs are as follows: they have one or more virulence genes

d. They are present in the genome of pathogenic members of a species, but absent in the nonpathogenic members; they are large

e. They typically have a different guanine plus cytosine (G + C) content than the rest of the bacterial genome

f. They are commonly associated with tRNA genes

g. They are often found with parts of the genome associated with mobile genetic elements; they often have genetic instability

h. They often represent mosaic structures with components acquired at different times.

Table - A Few Examples of the Very Large Number of Pathogenicity Islands of Human Pathogens

Genus/Species

PAI Name

Virulence Characteristics

Escherichia coli

PAI I536

Alpha hemolysin, fimbriae, adhesions, in urinary tract infections

Escherichia coli

PAI IJ96

Alpha hemolysin, P-pilus in urinary tract infections

Escherichia coli (EHEC)

O1#7

Macrophage toxin of enterohemorrhagic Escherichia coli (EHEC)

Salmonella typhimurium

SPI-1

Invasion and damage of host cells; diarrhea

Yersinia pestis

HPI/pgm

Genes that enhance iron uptake

Vibrio cholerae El Tor O1

VPI-1

Neuraminidase, utilization of amino sugars

Staphylococcus aureus

SCC mec

Methicillin and other antibiotic resistance

Staphylococcus aureus

SaPI1

Toxic shock syndrome toxin-1, enterotoxin

Enterococcus faecalis

NPm

Cytolysin, biofilm formation

General Microbiology Flashcard List

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