51. Microbiology | Bacteria I - Structure & Physiology

What are the key components of a bacterial cell envelope?

Cytoplasmic membrane, cell wall (peptidoglycan), and possibly periplasmic space, outer membrane, capsule, pili, flagella

What distinguishes Gram-positive from Gram-negative bacteria in terms of peptidoglycan?

Gram-positive: thick multilayered peptidoglycan; Gram-negative: thin 1-2 layered peptidoglycan

Which bacteria have teichoic acids in their cell wall?

Gram-positive bacteria

Where is lipopolysaccharide (LPS) found?

Outer membrane of Gram-negative bacteria

What color do Gram-positive bacteria stain?

Purple

What color do Gram-negative bacteria stain?

Pink/red

Name examples of Gram-positive bacteria

Staphylococcus aureus, Streptococcus pyogenes, Bacillus species

Name examples of Gram-negative bacteria

Escherichia coli, Neisseria gonorrhoeae, Pseudomonas aeruginosa

What is peptidoglycan composed of?

Alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) with amino acids for crosslinking

How does vancomycin affect the cell wall?

Prevents attachment of NAM/NAG

What do penicillins and beta-lactams inhibit?

Transpeptidation (crosslinking of peptidoglycan)

What is the role of lysozyme?

Breaks the bond between NAM and NAG

Why do Gram-positive bacteria stain purple?

Their thick peptidoglycan layer with techoic acids retains crystal violet-iodine complex

Why do Gram-negative bacteria stain pink?

Their thin peptidoglycan does not retain crystal violet and takes up safranin. 1º stain is washed out by decolorizer

Why can't Mycoplasma "walking pneumonia" be Gram stained?

They lack a cell wall

Why are beta-lactam antibiotics ineffective against Mycoplasma?

No peptidoglycan to target

What unique feature does acid-fast bacteria have in their envelope?

Mycolic acids (long-chain fatty acids) making the wall waxy + hydrophobic

What stain is used for acid-fast bacteria?

Ziehl-Neelsen stain

How do acid-fast bacteria appear with Ziehl-Neelsen stain?

Red/pink rods

What are examples of acid-fast bacteria?

Mycobacterium tuberculosis, Mycobacterium leprae

What is an alternative stain for acid-fast bacteria used in labs?

Auramine-Rhodamine fluorescent stain

Why does Treponema pallidum (syphilis) not Gram stain well?

Too thin; visualized with dark-field microscopy or silver stain

Why don't intracellular bacteria stain well?

Reside inside host cells; require special culture or immunostaining

What are bacterial capsules composed of?

Usually polysaccharide, sometimes polypeptide

What are examples of intracellular bacteria?

Chlamydia, Rickettsia (causes typhus), Legionella (causes pneumonia), Bartonella

What are functions of bacterial capsules?

Anti-phagocytic, adherence, biofilm formation, complement resistance

What does LPS consist of?

Lipid A (toxic), core polysaccharide, O-antigen

What is the pathogenesis of LPS?

Lipid A triggers cytokine storm, septic shock, fever, DIC, hypotension

What are bacterial endospores?

Highly resistant dormant structures formed by certain Gram-positive bacteria

What makes endospores resistant?

Low water content, high calcium dipicolinate

What triggers endospore formation?

Unfavorable conditions like nutrient depletion

What happens when conditions improve for a spore?

It germinates into a vegetative cell

Name aerobic/facultative gram (+) spore-forming bacteria

Bacillus anthracis, Bacillus cereus (food poisoning)

Name obligate anaerobic, gram (+) spore-forming bacteria

Clostridium botulinum, Clostridium tetani, Clostridium difficile, Clostridium perfringens (gas gangrene/food poisoning)

What kills spores in clinical settings?

Autoclaving at 121C, bleach-based disinfectants

What is transformation in bacteria?

Uptake of naked DNA from the environment

What is an example of transformation?

Griffith's experiment with Streptococcus pneumoniae

What is transduction?

Gene transfer via bacteriophage

What is an example of transduction?

Diphtheria toxin gene in Corynebacterium diphtheriae

What is an example of conjugation?

E. coli, Klebsiella

What is conjugation?

Plasmid transfer via direct contact (e.g., sex pilus)

Why is conjugation clinically important?

Spreads multidrug resistance via plasmids

What is an obligate aerobe?

Bacteria that require O2, found in lung infex

What are examples of obligate aerobes?

Mycobacterium tuberculosis, Bacillus spp

What are examples of obligate anaerobes?

Clostridium spp. i.e. C. tetani

What are examples of facultative anaerobes?

E. coli, S. aureus

What are examples of aerotolerant anaerobes?

Certain Streptococcus species

What is an obligate anaerobe?

Bacteria that cannot tolerate O2 common in deep wounds

What is a facultative anaerobe?

Bacteria that grow with or without O2

What is an aerotolerant anaerobe?

Do not use O2 but are not harmed by it

What is a microaerophile?

Requires low levels of O2, thrives in reduced O2 i.e. gastric mucosa

What are examples of microaerophiles?

Helicobacter pylori, campylobacter jejuni

What temperature range do mesophiles prefer?

Body temperature (~37C)

What pH do most pathogens prefer?

Near-neutral pH

What makes Helicobacter pylori unique in terms of pH?

It is adapted to acidic environments

What is seen in a Gram stain of mixed culture?

Purple clusters of Gram-positive cocci (i.e. Staphylococcus) and pink Gram-negative rods (i.e. E. coli)

What does Ziehl-Neelsen stain show?

Bright red/pink acid-fast rods on a blue background i.e. Mycobacterium Tb

What does a capsule stain show?

Pale halo around cells like Streptococcus pneumoniae

What does a spore stain show?

Green-stained spores inside pink Bacillus rods