1. Chapter 7
Microbial Growth

2. Growth
microbiologists usually study population growth (# cells in a bacterial population) rather than growth of individual cells
because cells are small in size

3. The Procaryotic Cell Cycle
cell cycle is sequence of events when a new cell divides into two.
most bacteria divide by binary fission
two pathways function during cycle
DNA replication and partition
cytokinesis

4. Figure 6.1

5. Chromosome Replication and Partitioning
most procaryotic chromosomes are circular
origin of replication – site at which replication begins
terminus – site at which replication is terminated
replisome (at the origin of replication) – group of proteins needed for DNA synthesis; parent DNA spools through the replisome as replication occurs
MreB – an actin homolog plays role in determination of cell shape and chromosome movement. (opposite poles of the cell)

6. Figure 6.3

7. Cytoskeletal Proteins - Role in Cytokinesis
process not well understood
protein MreB
similar to eucaryotic actin
plays a role in determination of cell shape and movement of chromosomes to opposite cell poles
protein FtsZ,
similar to eucaryotic tubulin - microtubules
plays a role in Z ring formation which is essential for septation
Divides the cell in two by constriction

8. Figure 6.4

9. DNA Replication in Rapidly Growing Cells
cell cycle completed in 20 minutes
40 minutes for DNA replication
20 minutes for septum formation and cytokinesis
look at timing-how can this happen?
Second, third or fourth round of replication can begin before first round of replication is completed

10. The Growth Curve
observed when microorganisms are cultivated in batch culture
culture incubated in a closed vessel with a single batch of medium
usually has four distinct phases

11. Fig. 6.6

12. Lag Phase cell synthesizing new components varies in length
e.g., to replenish spent materials
e.g., to adapt to new medium or other conditions
varies in length
in some cases can be very short or even absent

13. Exponential Phase also called log phase rate of growth is constant
population is most uniform in terms of chemical and physical properties during this phase
Culture is most sensitive to antibiotics
Metabolically most active

14. Stationary Phase total number of viable cells remains constant
may occur because metabolically active cells stop reproducing
may occur because reproductive rate is balanced by death rate
#cells dividing equals #cells dying

15. Possible reasons for entry into stationary phase
nutrient limitation
limited oxygen availability
toxic waste accumulation - acids

16. Starvation responses morphological changes
e.g., endospore formation
decrease in size, protoplast shrinkage, and nucleoid condensation
production of starvation proteins –
increased virulence – harder to kill
Strengthen peptidoglycan, chaperone protein prevent denaturation

17. Death Phase More and more cells are dying
# cells dying exceeds the # new cells formed

18. Generation time generation (doubling) time
time required for the population to double in size ((1cell to divide into two)
Varies depending on species of microorganism and environmental conditions
E.coli 20 minutes
M. leprae 14 days

19. The Influence of Environmental Factors on Growth
most organisms grow in fairly moderate environmental conditions
extremophiles
grow under harsh conditions that would kill most other organisms

21. Adaptations of thermophiles
protein structure stabilized by a variety of means
e.g., more H bonds
e.g., more proline
e.g., chaperones
histone-like proteins stabilize DNA
membrane stabilized by variety of means
e.g., more saturated, more branched and higher molecular weight lipids

22. Microbial growth is also influenced by pH.
Optimum growth pH 6.5 to 7.5
Acidophiles – like acidic pH
Sulfolobus – temperature 70oC
pH 2 (thermophile)
Alkalophiles – basic pH
Nitrosomonas – pH 8 to 8.8
Nitrosomonas – soil
Fungi – pH 5 to 6

23. pH
most acidophiles and alkalophiles maintain an internal pH near neutrality
The plasma membrane is impermeable to protons
some synthesize proteins that provide protection
e.g., acid-shock proteins – chaperone
Pump protons out

24. Osmosis – movement of water molecules from an area of high concentration of water to an area of low concentration of water through a selectively permeable membrane.
hypotonic – solute concentration lower outside the cell. Water moves into the cell.
Cell swells up and breaks down. Osmotic lysis.
Cell walls are damaged – bacteria undergo osmotic lysis.

27. Osmotic environment
Most bacteria cannot survive in hypertonic environment.
Plamolysis
High concentration of salt is used to preserve food products.
Halophiles – need high concentration of salt for growth ( at least 30% salt)
Salt lakes, dead sea
Halobacterium

29. Facultative halophiles –can grow in an environment that has 2 to 15% salt.
Do not need high concentration of salt for growth.
Grow in the presence or absence of salt
Staphylococcus aureus

30. Osmotolerant organisms
grow over wide ranges of water activity
many use compatible solutes to increase their internal osmotic concentration
solutes that are compatible with metabolism and growth – amino acids
some have proteins and membranes that require high solute concentrations for stability and activity

31. Oxygen Concentration need oxygen ignore oxygen < 2 – 10% oxygen
prefer
oxygen
oxygen is
toxic
Figure 6.22
thioglycolate

32. Basis of different oxygen sensitivities
oxygen easily reduced to toxic products
superoxide radical
hydrogen peroxide
aerobes produce protective enzymes
superoxide dismutase (SOD)
catalase

33. Oxygen requirement Obligate aerobes – need O2 for growth Micrococcus
Facultative anaerobe – can grow in the presence or absence of O2. it grows better in the presence of O2. E.coli
Obligate anaerobe – only grow in the absence of O2. Clostridium

34. Cells use O2 during cellular respiration.
Small amount of toxic O2 – super oxide free radical is made – damage to the structures
Obligate aerobes and facultative anaerobes – enzymes to neutralize the toxic O2
Super oxide dismutase (SOD)
Free radical – hydrogen peroxide (H2O2)
Catalase – H2O2 – water and oxygen
Aerotolerant anaerobes – tolerate O2 – do not use O2. Lactobacillus

35. Figure 6.24

36. Responses to low nutrient levels (oligotrophic environments)
organisms become more competitive in nutrient capture and use of available resources
morphological changes to increase surface area and ability to absorb nutrients
mechanisms to sequester certain nutrients
Group translocation
Caulobacter

37. Biofilms ubiquitous in nature
complex, slime enclosed colonies attached to surfaces
Layers of slime on rocks or other objects in water
Different species live together in a cocoon made up of slime
when form on medical devices such as implants (hip and knee) often lead to illness
can be formed on any surface

38. Biofilm formation – slime encased communities
Layers of slime on rocks
Surface conditioned with proteins
microbes reversibly attach to surface and release polysaccharides, proteins, and DNA – matrix – extracellular polymeric substances (EPS) – stable attachment
additional polymers are produced as biofilm matures
interactions occur among the attached organisms

39. Figure 6.28

40. Waste product of one microbe can be the energy source of another microbe.
DNA in the EPS can be taken up by some of the microbes - changes the genetic make up

41. Biofilm Microorganisms
the EPS and change in attached organisms’ physiology protects microbes from harmful agents
UV light, antibiotics, antimicrobials
when formed on medical devices, such as implants, often lead to illness
sloughing off of organisms can result in contamination of water phase above the biofilm such as in a drinking water system. Inner wall of pipes in the water supply

42. Biofilms
a mature biofilm is a complex, dynamic community of microorganisms
heterogeneity is differences in metabolic activity and locations of microbes
interactions occur among the attached organisms
exchanges take place metabolically, DNA uptake and communication

43. Figure 7.34 Competent To pick up DNA Peptides activate genes
That code for
Toxins to
Kill cells that are
Not competent

44. Biofilm microorganisms
extracellular matrix and change in attached organisms’ physiology protects them from harmful agents such as UV light and antibiotics
sloughing off of organisms can result in contamination of water phase above the biofilm such as in a drinking water system

45. Cell to Cell Communication Within the Microbial Populations
bacterial cells in biofilms communicate in a density-dependent manner called quorum sensing
produce small proteins that increase in concentration as microbes replicate (increase in number) and convert a microbe to a competent state. In this state
DNA uptake occurs, bacteriocins are released – kill cells that are not competent

46. Quorum Sensing
Minimum number cells have to be present for the regulation of certain genes
regulate the expression of bunch genes in a cell.
For this process to take place, there has to be minimum number of cells in the area.

47. Quorum Sensing Systems
processes regulated by quorum sensing involve host-microbe interactions
symbiosis – Vibrio fischeri and bioluminescence in squid – first discovered
Free living no light
Whole bunch of cells in the same area
pathogenicity and increased virulence factor production
DNA uptake for antibiotic resistance genes
Light organ, once the cell density increases
Cells give out light (flash light)

48. Figure 7.36a

49. Figure 7.36b
Catch its prey
Escape from
Predators
Flash light

50. Quorum Sensing
acylhomoserine lactone (AHL) is an autoinducer molecule produced by many gram-negative organisms (activating certain genes)
Few cells – AHL stays in the environment. As the cells reproduce, cell density incases. More AHL produced. diffuses across plasma membrane.once inside the cell it induces expression of target genes that regulate a variety of functions

51. Dental plaque formed on the surface of teeth is a biofilm
Different species of bacteria and such as S. mutans and candida albicans (yeast)