Chapter 6: Microbial Growth

How do bacteria grow?

  Not in size

Increase in population size

One cell divides into 2 new cells – binary fission

Binary fission

Binary fission:

Attachment of chromosome to p.m.; replication of DNA; new p.m. and cell wall laid down between the 2 chromosomes

This is the way that each new daughter cell gets one chromosome

Number of generations

Number of cells Log of # of cells10X = #

0 1 01 2 0.32 4 0.63 8 0.94 16 1.25 32 1.56 64 1.87 128 2.18 256 2.49 512 2.710 1024 3.011 2048 3.312 4096 3.6

How can we describe growth?

2n = no. of cells in n generation

Generation time

Nt = N0 x 2n

Growth problemsIf Staphylococcus aureus has a doubling time

(generation time) of 30 minutes and 5 hours have passed, how many generations have been produced?

a. How many 30 minute time chunks are in 5 hours?

= 10

ANSWER: 10 generations

Growth problemsIf Staphylococcus aureus has a generation time

of 30 minutes and 5 hours have passed, how many bacteria will be present at the end of the time period?

a. We have already determined that 10 generations will occur.

b. 2n = # cells at n generationc. 210 = # cells at the 10th generation

ANSWER = 1024 cells

Growth problemsIf Staphylococcus aureus has a generation time

of 30 minutes and 5 hours have passed, how many bacteria will be present at the end of the time period if we start with 3,000 cells? Nt = N0 x 2n

Nt = 3,000 x 210

= 3,072,000 cells at the end of 5 hours

Growth curve: lag, log, stationary, and death phases

What occurs in each?

How can population size be counted?(Advantages and disadvantages of each

method)

1. Direct methods

A. Microscopic count with hemacytometer/

Petroff Hauser counting chamber

Petroff Hauser counting chamber

B. Plate counts – dilution series and plates

C. Filtration

D. Coulter counter/flow cytometer/Fluorescence activated cell sorter (FACS)

2. Indirect methodsA. Dry weight

B. Metabolic activity

C. Turbidity

Turbidity

Growth requirements of microbesA. Temperature:

ThermophilesMesophilesPsychrophiles

B. pH: acidophilesC. Osmotic pressure (# of solutes in solution)

HalophilesD. Oxygen:

Types:Obligate aerobeFacultative anaerobeObligate anaerobeAerotolerant anaerobeMicroaerophilic

Enzymes needed to survive in presence of oxygen:

CatalasePeroxidaseSuper oxide dismutase (SOD)

E. Nutrients

C, N, P, S elements neededMg, Fe, etc. trace elements neededMedia:

Defined or complexSelective vs. differentialSpecial

Mannitol salt agar – selective medium

Blood agar – differential medium

The End