Microbiology: Bacterial Growth, Structure, and Medical Importance

Practice questions covering bacterial growth phases, morphology, surface area ratios, cell wall composition (peptidoglycan), Gram and Acid-fast staining procedures, and mechanisms of antibiotic action and resistance.

What characterizes the lag phase of the bacterial growth curve?

Bacteria adapt to the new environment; cells are non-dividing (teal/blue) but metabolically active.

What is the log (exponential) phase in bacterial growth?

A phase where cells grow and divide rapidly through binary fission (green), causing an exponential increase in viable cell density.

In bacterial growth, what occurs during the stationary phase?

The growth rate slows as nutrients become limited and waste accumulates; the number of new cells produced equals the number of dying cells, leading to a plateau.

What is the meaning of the acronym GASP in microbiology?

Growth-Associated Stationary Phase.

How do GASP cells function during the long-term stationary phase?

They possess genetic mutations allowing them to adapt to severe nutrient limitation and accumulate even as the majority of the population declines.

What is the definition of generation time (doubling time)?

The specific amount of time required for a microbial population to double in number through one round of binary fission.

How does the Surface Area to Volume (SA:V) ratio affect nutrient uptake?

A higher SA:V ratio allows for more efficient nutrient uptake and waste removal.

As an organism or cell gets larger, what happens to its SA:V ratio?

Its volume increases faster than its surface area, resulting in a lower SA:V ratio.

Which bacterium is identified as growing the fastest with a doubling time of $10\,\text{min}$?

Vibrio natriegens

Why does Vibrio natriegens grow faster than Thiomargarita magnifica in terms of SA:V ratio?

Vibrio natriegens has a Higher SA:V ratio, whereas Thiomargarita magnifica has a Lower SA:V ratio.

What immune evasion strategy does Uropathogenic E. coli (UPEC) use to avoid phagocytosis?

It intentionally elongates into filaments (filamentation) to become too large for phagocytes to engulf.

What is the role of the $sulA$ gene in UPEC filamentation?

As part of the SOS response, the $sulA$ gene is expressed to produce the SulA protein, which inhibits cell division.

How does the SulA protein inhibit bacterial cell division?

It binds directly to the essential cell division protein FtsZ, preventing it from polymerizing and forming the Z-ring.

Which protease is responsible for degrading FtsZ during the death phase?

$$ClpXP$$

Why is a bacterial culture typically considered 'too old' for identifying morphology after $24+$ hours?

Once in the death phase, cells begin to lyse or break open due to nutrient lack and waste buildup, destroying uniform shape and structure.

Which bacterial shape is associated with terms like 'comma' or 'curved rod' and uses the $crvA$ gene for its morphology?

Vibrio cholerae

How does the protein CrvA create cell curvature in Vibrio cholerae?

It is a periplasmic element that assembles into filaments along the inner curvature, causing the cell wall to grow unevenly.

Name three pathogenic spirochetes and the diseases they cause.

Leptospira (Leptospirosis), Borrelia (Lyme disease & Relapsing Fever), and Treponema pallidum (Syphilis).

How does Helicobacter pylori navigate the stomach's mucin layer?

It releases urease to neutralize stomach acid, which liquefies the gel-like mucin layer, allowing a 'drill-like swim' motion with potent flagella.

What volatile organic compound produced by Streptomyces is responsible for the earthy scent of soil after rain?

$$geosmin$$

Name three antibiotics produced by the filamentous genus Streptomyces.

$streptomycin$, $chloramphenicol$, and $tetracycline$.

What are the two sugar monomers that form the glycan backbone of peptidoglycan?

N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

What type of bond links NAM and NAG monomers in the peptidoglycan backbone?

$\beta$-(1,4) glycosidic bonds

Which D-form amino acids are typically found in the peptide chains of peptidoglycan?

D-alanine (D-ala) and D-glutamic acid (D-glu).

In a Gram stain, what color do Gram-positive and Gram-negative bacteria appear?

Gram-positive appear purple (Crystal Violet); Gram-negative appear pink (Safranin).

Why do Gram-negative bacteria lose the crystal violet stain during decolorization?

The alcohol dissolves the high-lipid outer membrane and dehydrates the thin peptidoglycan layer, allowing the crystal violet-iodine complex to wash away.

Why does the Gram stain fail to work on Mycobacterium and Nocardia?

They possess a waxy, hydrophobic mycolic acid layer that resists the penetration of basic dyes.

What are the primary stain and decolorizer used in the Ziehl-Neelsen Acid-fast stain?

Primary stain: $carbolfuchsin$ (with phenol); Decolorizer: $acid-alcohol$ ($3\%\,HCl$ in ethanol).

Why is the Gram stain not the most accurate method for diagnosing yeast like Candida?

Yeast are eukaryotes with cell walls made of chitin, beta-glucan, and mannoproteins; they lack the peptidoglycan layer found in bacteria.

How does the enzyme lysozyme kill bacteria?

It cleaves the $\beta$-(1,4)-glycosidic bonds between NAM and NAG in the peptidoglycan layer, causing cell lysis.

Which enzyme is inactivated by penicillin to block peptidoglycan synthesis?

$$transpeptidase$$

What is the mechanism of resistance in Methicillin-resistant Staphylococcus aureus (MRSA)?

The $mecA$ gene encodes $PBP2a$, a transpeptidase with a unique active site that has a low affinity for $\beta$-lactam antibiotics.

How does Staphylococcus aureus resist lysozyme?

It performs O6-acetylation of NAM residues via the enzyme $OatA$, creating steric hindrance that blocks lysozyme binding.