microbes lecture 6-7

Question 1

What do microbes need to grow?

Answer 1

Appropriate environmental conditions and nutrients.

Question 2

What is binary fission?

Answer 2

How bacteria reproduce. Chromosome attaches to cell wall in full size cell. DNA replicates to give two chromosomes, attachment site duplicates, chromosomes separate, a septum to divide the cytoplasm into two forms. New walls and membranes form two new cells which are seperated.

Question 3

Bacteria multiplies via … … this results in the cells doubling which is an … increase.

Answer 3

Binary fission, exponential.

Question 4

What is the time between each division of microbes known as? What is the average amount of time

Answer 4

Generation time (GT), 30 minutes.

Question 5

Equation for predicted value of a microbial population growth in a time period?

Answer 5

LogNt = 0.301n + logN0 
Nt = number of cells after the  time period
N0 = number of cells at beginning of growth period (inoculum).
n = the time period divided by the generation time.

Question 6

How many cells of E. coli after 24 hours of logarithmic growth, starting with inocumlum of cells of 100 cells in a batch tank, if E.coli has a generation time of 20 minutes?

Answer 6

Log23.672 or 4.7 x 10^23 cells. refer to lecture 6/7 slide 8 if confused.

Question 7

What is the lag phase of bacterial growth?

Answer 7

First stage where cells are growing & increasing in size, but not reproducing (adapting to new environment).

Question 8

What is the logarithmic phase of bacterial growth?

Answer 8

Second stage: cells multiply rapidly at a constant rate. Follows the first rule of kinetics; slope and length of ‘log’ phase depend on nutrients and environmental conditions.

Question 9

What is the stationary phase of bacterial growth?

Answer 9

Third phase: growth rate decreases to point where no net increase in cell numbers – owing to shortage of nutrients and build up of waste products / environment change (e.g. pH)

Question 10

What is the decline/ death phase?

Answer 10

Forth and final phase: rate of cell death > rate of cell multiplication (owing to nutrient shortage / build- up of waste products).

Question 11

What is the difference between bacteria and yeast?

Answer 11

Bacteria are prokaryotic which is 1 to 10 µm in size and has no nucleus. Yeast are eukaryotic 20 - 100 µm in size and has nucleaus.

Question 12

Difference and similarities between yeasts and moulds?

Answer 12

They are both eukaryotic but moulds are multicellular and yeast is unicellular.

Question 13

how does mould grow?

Answer 13

Moulds grow asexually by increasing length, diameter and number of hyphae and hyphal branches.

Question 14

How is mould growth calculated?

Answer 14

plot of colony diameter over time.

Question 15

How does yeast grow?

Answer 15

Multiply by budding – bud protrudes from mother cell and becomes a daughter cell;
Cells may separate to form two full size yeast cells, or stay together and form a chain

Question 16

What can the growth rates of yeast be compared to?

Answer 16

Bacteria

Question 17

How are microbes used in food production.

Answer 17

To make fermented dairy products (e.g. cheese), fermented drinks like beer, fermented vegetables like kimchi, bread, fermented meat like some sausages and fermented soy products like soy sauce.

Question 18

Types of non-microbial food spoilage?

Answer 18

Physical – e.g. dehydration causing wilting of vegetables;
Chemical – e.g. oxidation of fatty acids (rancidity), e.g. browning.
Infestation – insects, rodents

Question 19

Ways microbes spoil food?

Answer 19

Direct effect of growth of microbes which can cause things like slime
Effect of microbial enzymes on the food

Question 20

What is controversial about food spoilage?

Answer 20

Spoilage can be a matter of opinion such as very strong camembert some might think it is spoiled.

Question 21

What do microorganisms need to spoil food?

Answer 21

Microorganisms must gain access to the food. Physical and chemical conditions must favour growth of contaminant microorganisms.
Food must be stored at appropriate temperature for sufficient length of time for population to increase sufficiently to cause spoilage

Question 22

When is microbe spoilage usually detected?

Answer 22

Spoilage usually detected at 10^6 – 10^8 cells / g, /ml, or/cm2
Varies with type of spoilage and micro-organism.

Question 23

Two of the same food products are stored in the exact temperature and conditions but one spoils sooner why is this?

Answer 23

Because one has higher initial bacteria.

Question 24

Order growth rate of bacteria, yeasts and moulds?

Answer 24

bacteria grows the fastest, then yeast, then mould.

Question 25

When are moulds a major problem with examples?

Answer 25

in foods stored for relatively longer time with conditions unfavourable to bacteria and yeasts (e.g. breads, hard cheeses).

Question 26

What determines dictate which species of microbe multiply and predominate?

Answer 26

Prevalent conditions (e.g. pH, aw, nutrients, atmosphere…)

Question 27

What do predominant microorganisms do in food and what does this lead to?

Answer 27

Predominant organisms produce metabolites and alter the food environment.
This may provide opportunity for another species to grow rapidly and further alter environment…
……Which may permit 3rd species to grow rapidly.
Given sufficient time, the predominant microbial types, and the nature of the spoilage, can change

Question 28

Microbial succession in milk?

Answer 28

Lactococcus (can metabolise lactose; lowers pH), then bacillus (aciduric; can metabolise proteins; raises pH) and finally pseudomonas (metabolises proteinaceous cpds; raises pH further).

Question 29

What two types of organism do refrigeration temperatures prevent growth of?

Answer 29

Thermophiles and mesophilic organisms.

Question 30

What organsisms are important in spoilage od chilled foods?

Answer 30

Psychrophiles and psychotrophs which are more temperature versatile.