Microbio Week 7 Quiz

What are the indicator and positive color result for the Phenol Red Broth (PRB) fermentation test?

indicator is phenol red

turns yellow when acidic fermentation products are present

What reagent indicator is used and what indicates a positive result in the Starch Agar Test?

Gram’s Iodine is used; clear zones/white patches around growth where starch was hydrolyzed=+ result

What reagent is added and what indicates a positive color result in the Methyl Red (MR) Test?

Methyl Red indicator is added; turns red in presence of strong mixed-acid fermentation products=+ result

What metabolite is detected and what color indicates a positive result in the Voges-Proskauer (VP) Test?

VP-A and VP-B reagents detect acetoin production; red=+ result

What color and chemical condition indicate a positive result for the Simmons Citrate Agar test?

Blue color=+ result under alkaline (basic) conditions resulting from citrate utilization

What is bacterial generation time and what is its mathematical equation?

Generation Time (G) is the time required for a bacterial cell to divide into two cells

G=delta t/n

How does a spectrophotometer estimate cell density in a bacterial culture?

measures Optical Density (OD) or absorbance by quantifying amount of light scattered by cells suspended in the culture

What are the four main phases of a standard bacterial growth curve?

Lag Phase

Exponential (Log) Phase

Stationary Phase

Death (Decline) Phase

What main physiological activity occurs during the bacterial lag phase?

Bacteria adjust to environment accelerating metabolic activity to synthesize macromolecules and enzymes without cell division

What primary factors cause a bacterial culture to enter the stationary phase?

Nutrient depletion and accumulation of toxic metabolic waste products cause the rate of cell division to equal the rate of cell death

What is the primary advantage of plotting a bacterial growth curve on a semi-logarithmic scale?

it converts exponential growth data into a straight line, making it significantly easier to plot wide ranges of values and calculate the growth rate slope

How did Louis Pasteur’s swan-neck flask experiments disprove the theory of spontaneous generation?

he showed that sterilized broth remains free of microbes as long as dust particles were trapped in the curved neck, proving microbes come from airborne contamination rather than nonliving matter

How are electromagnetic radiation wavelength, energy, and penetrating power related?

shorter radiation wavelengths carry higher energy and possess greater penetrating power than longer wavelengths

What primary mechanism causes cellular and DNA damage during exposure to ionizing radiation?

Ionizing radiation ionizes water into reactive free radicals that causes single and double strand breaks in bacterial DNA

Which UV radiation spectrum has the highest biocidal activity and why?

UVC (100 nm - 280 nm) has the strongest biocidal activity because its wavelengths correspond to the optimal absorption peak and bacterial DNA

What specific DNA lesion is induced in bacteria in ultraviolet (UV) light exposure?

UV radiation causes adjacent pyrimidine bases to covalently bond, forming thymine dimers that distort the DNA double helix

How does light repair (photoreactivation) repair UV-damaged bacterial DNA?

visible light activates photolyase enzymes that directly break the covalent bonds of thymine dimers to restore normal DNA structure

What sequence of enzymatic steps occurs during dark repair (excision repair) of damaged DNA?

endonucleases cut out the damaged DNA segment, DNA polymerase fills in the missing nucleotides, and DNA ligase seals the backbone

Why is E. coli strain XL2-Blue used in the UV radiation experiment?

because it is a mutant strain lacking functional DNA repair enzymes, serving as a highly sensitive control for UV radiation damage

What is the primary difference between antiseptics and disinfectants?

antiseptics are chemical germicides formulated for application on living tissue; disinfectants are used on inanimate objects and surfaces

What is the functional difference between bacteriostatic and bactericidal agents?

Bacteriostatic agents temporarily inhibit bacterial growth and metabolic activity; bactericidal agents kill bacterial cells

How is Decimal Reduction Time (DRT) defined?

it is the time required for a chemical agent or process to kill 90% of a test bacterial population at a specific concentration

What genus of soil bacteria produces nearly 70% of naturally derived antibiotics?

Streptomyces

a genus of soil-dwelling actinomycetes producing nearly 70% of all natural antibiotics

What agar medium is used for the Kirby-Bauer disk diffusion test and what are its key features?

Mueller-Hinton Agar (MHA) is used because it allows antimicrobial drugs to diffuse freely and contains starch to absorb microbial toxins

What is Minimum Inhibitory Concentration (MIC) and what does a lower MIC value indicate?

MIC is the lowest concentration of an antimicrobial drug required to inhibit visible bacterial growth; a lower MIC indicates greater antimicrobial potency