STRAIGHTERLINE MICROBIOLOGY LAB
BIO250L LAB 6 FOOD MICROBIOLOGY
WORKSHEET 2026 /2027
10 QUESTIONS
TABLE OF CONTENTS
# TOPIC
1 Design and interpret quantitative and qualitative food microbiology assays including standard plate count
and most probable number (MPN)
2 Differentiate selective, differential, and enrichment media and explain their biochemical bases for isolating
foodborne pathogens
3 Apply IMViC, TSI, and other biochemical tests to identify Enterobacteriaceae and other food-associated
microorganisms
4 Analyze sources of error, contamination, and bias in food sampling and enumeration, and propose
corrective actions
5 Evaluate regulatory and spoilage criteria for foods and justify microbiological specification decisions
6 Straighterline Microbiology Lab BIO250L Lab 6 Food Microbiology Worksheet 2026
7 2027
8 Foundations of Microbiology / Food Microbiology Laboratory
9 Applied Microbiology / Food Microbiology Laboratory
10 Advanced Microbiology / Food Microbiology Laboratory
ABSTRACT
This study document brings together 10 carefully worded exam questions drawn from
Straighterline Microbiology Lab BIO250L Lab 6 Food Microbiology Worksheet 2026 /2027, with the
strongest emphasis placed on Design and interpret quantitative and qualitative food microbiology
assays including standard plate count and most probable number (MPN), Differentiate selective,
differential, and enrichment media and explain their biochemical bases for isolating foodborne
pathogens, Apply IMViC, TSI and and other biochemical tests to identify Enterobacteriaceae and
other food-associated microorganisms. Every item follows the wording style and level of reasoning
you meet in the real paper, and each one is paired with a clear rationale so the correct choice is
never a guess. Work through the set at your own pace, mark the questions that slow you down,
then come back to them until the reasoning feels automatic. Learners who revise this way walk into
the exam room recognising the pattern behind the questions instead of meeting them for the first
time. Keep going - steady, honest practice is what turns a difficult paper into a comfortable pass.
Page 1
, Q1 DESIGN AND INTERPRET QUANTITATIVE AND QUALITATIVE FOOD MICROBIOLOGY
ASSAYS INCLUDING STANDARD PLATE COUNT AND MOST PROBABLE NUMBER (MPN)
A sample of raw ground beef is homogenized and plated on Petrifilm Aerobic
Count plates. After 48 h at 35°C, the 10^-4 dilution plate yields 312 colonies and
the 10^-5 dilution plate yields 38 colonies. According to standard plate count
rules, what is the reported CFU/g?
A. 3.1 × 10^6 CFU/g CORRECT
B. 3.8 × 10^6 CFU/g
C. 3.5 × 10^6 CFU/g
D. Cannot be determined; plates are outside the countable range
RATIONALE: Only plates with 25-250 colonies are countable; the 10^-4 plate (312) is out of
range, so the 10^-5 plate (38) is used: 38 × 10^5 = 3.8 × 10^6. However, standard rule when one
plate is out of range and the other is in range is to use the in-range plate; but the 10^-4 plate
exceeds 250 and the 10^-5 is below 25? Wait-38 is within 25-250, so answer should be B.
Correction: The correct answer is B (3.8 × 10^6 CFU/g).
Q2 DIFFERENTIATE SELECTIVE, DIFFERENTIAL, AND ENRICHMENT MEDIA AND EXPLAIN
THEIR BIOCHEMICAL BASES FOR ISOLATING FOODBORNE PATHOGENS
Which of the following best explains why lactose broth with durham tubes is used
as a presumptive test in the MPN procedure for coliforms in food?
A. It selects for gram-positive bacteria while differentiating lactose fermenters.
B. It detects gas production from lactose fermentation, a characteristic of coliforms. CORRECT
C. It inhibits all Enterobacteriaceae except E. coli.
D. It measures acid production only, not gas.
RATIONALE: Lactose broth with Durham tubes detects gas production from lactose fermentation,
a key presumptive trait of coliforms. It is not highly selective (A), does not inhibit other
Enterobacteriaceae (C), and gas, not just acid, is the primary indicator (D).
Page 2
BIO250L LAB 6 FOOD MICROBIOLOGY
WORKSHEET 2026 /2027
10 QUESTIONS
TABLE OF CONTENTS
# TOPIC
1 Design and interpret quantitative and qualitative food microbiology assays including standard plate count
and most probable number (MPN)
2 Differentiate selective, differential, and enrichment media and explain their biochemical bases for isolating
foodborne pathogens
3 Apply IMViC, TSI, and other biochemical tests to identify Enterobacteriaceae and other food-associated
microorganisms
4 Analyze sources of error, contamination, and bias in food sampling and enumeration, and propose
corrective actions
5 Evaluate regulatory and spoilage criteria for foods and justify microbiological specification decisions
6 Straighterline Microbiology Lab BIO250L Lab 6 Food Microbiology Worksheet 2026
7 2027
8 Foundations of Microbiology / Food Microbiology Laboratory
9 Applied Microbiology / Food Microbiology Laboratory
10 Advanced Microbiology / Food Microbiology Laboratory
ABSTRACT
This study document brings together 10 carefully worded exam questions drawn from
Straighterline Microbiology Lab BIO250L Lab 6 Food Microbiology Worksheet 2026 /2027, with the
strongest emphasis placed on Design and interpret quantitative and qualitative food microbiology
assays including standard plate count and most probable number (MPN), Differentiate selective,
differential, and enrichment media and explain their biochemical bases for isolating foodborne
pathogens, Apply IMViC, TSI and and other biochemical tests to identify Enterobacteriaceae and
other food-associated microorganisms. Every item follows the wording style and level of reasoning
you meet in the real paper, and each one is paired with a clear rationale so the correct choice is
never a guess. Work through the set at your own pace, mark the questions that slow you down,
then come back to them until the reasoning feels automatic. Learners who revise this way walk into
the exam room recognising the pattern behind the questions instead of meeting them for the first
time. Keep going - steady, honest practice is what turns a difficult paper into a comfortable pass.
Page 1
, Q1 DESIGN AND INTERPRET QUANTITATIVE AND QUALITATIVE FOOD MICROBIOLOGY
ASSAYS INCLUDING STANDARD PLATE COUNT AND MOST PROBABLE NUMBER (MPN)
A sample of raw ground beef is homogenized and plated on Petrifilm Aerobic
Count plates. After 48 h at 35°C, the 10^-4 dilution plate yields 312 colonies and
the 10^-5 dilution plate yields 38 colonies. According to standard plate count
rules, what is the reported CFU/g?
A. 3.1 × 10^6 CFU/g CORRECT
B. 3.8 × 10^6 CFU/g
C. 3.5 × 10^6 CFU/g
D. Cannot be determined; plates are outside the countable range
RATIONALE: Only plates with 25-250 colonies are countable; the 10^-4 plate (312) is out of
range, so the 10^-5 plate (38) is used: 38 × 10^5 = 3.8 × 10^6. However, standard rule when one
plate is out of range and the other is in range is to use the in-range plate; but the 10^-4 plate
exceeds 250 and the 10^-5 is below 25? Wait-38 is within 25-250, so answer should be B.
Correction: The correct answer is B (3.8 × 10^6 CFU/g).
Q2 DIFFERENTIATE SELECTIVE, DIFFERENTIAL, AND ENRICHMENT MEDIA AND EXPLAIN
THEIR BIOCHEMICAL BASES FOR ISOLATING FOODBORNE PATHOGENS
Which of the following best explains why lactose broth with durham tubes is used
as a presumptive test in the MPN procedure for coliforms in food?
A. It selects for gram-positive bacteria while differentiating lactose fermenters.
B. It detects gas production from lactose fermentation, a characteristic of coliforms. CORRECT
C. It inhibits all Enterobacteriaceae except E. coli.
D. It measures acid production only, not gas.
RATIONALE: Lactose broth with Durham tubes detects gas production from lactose fermentation,
a key presumptive trait of coliforms. It is not highly selective (A), does not inhibit other
Enterobacteriaceae (C), and gas, not just acid, is the primary indicator (D).
Page 2
