MBIO 3401 LAB EXAM 2 QUESTIONS AND ANSWERS
2026
What are the phases in bacterial growth curve? What happens in each phase? -
Answers - -lag phase: no increase in cell growth; organisms get used to the new
environment
-exponential/log phase: cell growth and cell division happen at a fast rate and at regular
time intervals; nutrients are abundant; doubles in size
-stationary phase: the level of waste products build up to concentrations that become
toxic; bacterial cells slow down their metabolism and do not divide anymore--
ENDOSPORES
-death phase: cells die at a rate that exceeds any cell division and the number of cells
starts to decline exponentially with time
What is generation time? - Answers - The time for one bacterium to multiply into two;
varies from organism to organism
Draw growth curve - Answers - Page 68
X-axis: time in units of minutes and hours
Y-axis: number of cells in log units
How do bacteria replicate? - Answers - Binary fission
What are the different methods that we can use to measure bacterial growth? - Answers
- -Standard plate counts are used to measure the number of viable cells in a bacterial
culture. This technique is done by taking an aliquot of the bacterial culture at every time
point and the number of viable cells is determined. A serial dilution must first be
performed and then each dilution is plated.
-Light transmittance: a spectrophotometer uses light to analyze turbidity. A spec allows
you to pick out a specific wavelength of light and then to measure absorbance or
transmittance of light through the substance. Absorbance is the amount of light that's
absorbed by the culture, and transmittance is the amount of light that's able to reach the
detector after traveling through the culture.
Define selective and differential media - Answers - -selective: allows the organism of
interest to grow, while inhibiting the growth of others (ex. MAC, MSA)
, -differential: contains substances that allow certain groups of bacteria to appear
differently than others when grown on the same media; differentiates between two
groups of bacteria
MAC (MacConkey's agar) - Answers - **crystal violet and bile salts**
**lactose and neutral**
-growth of gram-negative bacteria
-bile salts aid in the isolation of enteric bacteria, INHIBITS GRAM POSITIVE
-contains lactose and neutral red, a pH indicator; if capable of utilizing lactose, then it
produces acidic end products --> turns the colonies **red**
-non lactose fermenters do not change the pH --> colonies remain transparent
MSA (Manitol salt agar) - Answers - **mannitol and phenol red**
-growth of gram-positive bacteria
-contains high levels of salt-- INHIBITS GRAM NEGATIVE
-when mannitol is fermented, the pH drops and the medium around the colony turns
**yellow**; the medium around non-mannitol fermenters remains pink
KIA media (Exercise 18) - Answers - -differential media which serves both as a
determiner of carbohydrate fermentation patterns and gas production; useful for
deteriming whether a gram-negative rod is a member of the family Enterobacteriaceae
-tells us utilization of peptone and gas production
-production of acid by carbohydrate fermentation turns the medium **yellow** due to the
**pH indicator phenol red**
**How to interpret KIA results** - Answers - 1. yellow slant/yellow deep = (A/A) glucose
and lactose are fermented; if the butt has bubble in it or is displaced, the tube is gas
positive
2. red slant/yellow deep = (K/A) glucose, but not lactose is fermented
3. red slant/black deep = (K/A) glucose but not lactose is fermented, and H2S is
produced. The deep is acid (yellow)
2026
What are the phases in bacterial growth curve? What happens in each phase? -
Answers - -lag phase: no increase in cell growth; organisms get used to the new
environment
-exponential/log phase: cell growth and cell division happen at a fast rate and at regular
time intervals; nutrients are abundant; doubles in size
-stationary phase: the level of waste products build up to concentrations that become
toxic; bacterial cells slow down their metabolism and do not divide anymore--
ENDOSPORES
-death phase: cells die at a rate that exceeds any cell division and the number of cells
starts to decline exponentially with time
What is generation time? - Answers - The time for one bacterium to multiply into two;
varies from organism to organism
Draw growth curve - Answers - Page 68
X-axis: time in units of minutes and hours
Y-axis: number of cells in log units
How do bacteria replicate? - Answers - Binary fission
What are the different methods that we can use to measure bacterial growth? - Answers
- -Standard plate counts are used to measure the number of viable cells in a bacterial
culture. This technique is done by taking an aliquot of the bacterial culture at every time
point and the number of viable cells is determined. A serial dilution must first be
performed and then each dilution is plated.
-Light transmittance: a spectrophotometer uses light to analyze turbidity. A spec allows
you to pick out a specific wavelength of light and then to measure absorbance or
transmittance of light through the substance. Absorbance is the amount of light that's
absorbed by the culture, and transmittance is the amount of light that's able to reach the
detector after traveling through the culture.
Define selective and differential media - Answers - -selective: allows the organism of
interest to grow, while inhibiting the growth of others (ex. MAC, MSA)
, -differential: contains substances that allow certain groups of bacteria to appear
differently than others when grown on the same media; differentiates between two
groups of bacteria
MAC (MacConkey's agar) - Answers - **crystal violet and bile salts**
**lactose and neutral**
-growth of gram-negative bacteria
-bile salts aid in the isolation of enteric bacteria, INHIBITS GRAM POSITIVE
-contains lactose and neutral red, a pH indicator; if capable of utilizing lactose, then it
produces acidic end products --> turns the colonies **red**
-non lactose fermenters do not change the pH --> colonies remain transparent
MSA (Manitol salt agar) - Answers - **mannitol and phenol red**
-growth of gram-positive bacteria
-contains high levels of salt-- INHIBITS GRAM NEGATIVE
-when mannitol is fermented, the pH drops and the medium around the colony turns
**yellow**; the medium around non-mannitol fermenters remains pink
KIA media (Exercise 18) - Answers - -differential media which serves both as a
determiner of carbohydrate fermentation patterns and gas production; useful for
deteriming whether a gram-negative rod is a member of the family Enterobacteriaceae
-tells us utilization of peptone and gas production
-production of acid by carbohydrate fermentation turns the medium **yellow** due to the
**pH indicator phenol red**
**How to interpret KIA results** - Answers - 1. yellow slant/yellow deep = (A/A) glucose
and lactose are fermented; if the butt has bubble in it or is displaced, the tube is gas
positive
2. red slant/yellow deep = (K/A) glucose, but not lactose is fermented
3. red slant/black deep = (K/A) glucose but not lactose is fermented, and H2S is
produced. The deep is acid (yellow)
