Lan practical 1 micro practice

What indicates a pellicle growth in broth cultures?

A film or skin at the top of the broth.

How can you identify sediment in a broth culture?

Growth settled at the bottom of the tube.

What does flocculent growth look like?

Clumps suspended in the broth.

Why does a pellicle indicate aerobic metabolism?

Bacteria cluster at the oxygen-rich surface.

If a broth is evenly turbid throughout, what two categories could it be?

Facultative anaerobe or aerotolerant anaerobe.

Why do obligate anaerobes only grow at the bottom of a tube?

They lack enzymes to detoxify oxygen radicals.

Why is flocculent growth sometimes compared to 'snow in a snow globe'?

Because the clumps swirl when shaken.

Which tube shows obligate aerobes in a fluid thioglycollate medium?

Growth only at the top.

What chemical in fluid thioglycollate medium reduces oxygen?

Sodium thioglycolate.

What indicates the presence of oxygen in fluid thioglycollate medium?

Resazurin, which turns pink when oxygen is present.

Why does oxygen kill obligate anaerobes?

They lack superoxide dismutase (SOD) and catalase, causing reactive oxygen species to build up.

If an organism prefers low oxygen but does not require it, what is it classified as?

Microaerophile.

Why do facultative anaerobes grow faster at the top of the broth?

They prefer aerobic respiration for higher ATP yield.

What bacteria are more likely to be resistant to penicillin: Gram-positive or Gram-negative?

Gram-negative, due to their outer membrane blocking entry.

Why does penicillin usually work well on Gram-positive bacteria?

Because it targets peptidoglycan cross-linking without an outer membrane barrier.

What does a large zone of inhibition mean in the Kirby-Bauer test?

The bacteria are susceptible to the antibiotic.

What does no zone of inhibition indicate?

The bacteria are resistant to the antibiotic.

Why is Mueller-Hinton agar used in Kirby-Bauer testing?

It ensures even diffusion of antibiotics and consistent growth of bacteria.

What is the typical growth range of mesophiles?

20–45 °C; optimum around 37 °C.

What is flocculent growth in broth culture?

Clumped flakes suspended in broth.

Why do we dilute cultures before plating?

To obtain countable colonies (30–300).

If you plate 0.1 mL of a 10⁻⁴ dilution and count 50 colonies, what is the CFU/mL of the original?

50 ÷ 0.1 mL × 10⁴ = 5 × 10⁶ CFU/mL.

What is the purpose of using sterile saline or broth in dilutions?

To disperse cells evenly without providing nutrients.

What are psychrophiles?

Microorganisms that grow best at refrigerator temperatures.

What temperature range do thermophiles thrive in?

60–80 °C.

What is the final electron acceptor in fermentation?

An organic molecule like pyruvate or acetaldehyde.

Why does fermentation only produce 2 ATP per glucose?

Only glycolysis occurs; no electron transport chain (ETC) or Krebs cycle.

What does the term 'fastidious' mean for bacteria?

Requires special growth factors or nutrients.

Why are mesophiles the most common pathogens?

Their optimum temperature aligns with human body temperature.

What happens if too much oil is used for oil immersion microscopy?

The image becomes blurry, and the lens can get contaminated.

What do sociable clumps or flakes suspended in broth indicate in bacterial growth?

Flocculent growth.

What is the normal growth temperature for hyperthermophiles?

Above 80 °C.

Why are psychrotrophs significant in food spoilage?

They can grow in both cold (refrigerator) and moderate temperatures.

How do you classify an obligate anaerobe based on oxygen tolerance?

It grows only at the bottom of a growth medium without oxygen.

What type of growth pattern is indicated by a colony with a knob-like center?

Umbonate growth.

What tool is used to streak an agar slant in microbiology labs?

Inoculating loop.