1 Microbial growth: prokaryotic cell cycle and growth curve Chapter 6

2 Growth Increase in cellular constituents that may result in: –increase in cell number binary fission –increase in cell size coenocytic microorganisms have nuclear divisions that are not accompanied by cell divisions Microbiologists usually study population growth rather than growth of individual cells

3 The Procaryotic Cell Cycle Cell cycle is sequence of events from formation of new cell through the next cell division Two pathways function during cycle –replication of DNA and partition –cytokinesis (cell separation)

Copyright © McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 Figure 6.1

5 The Cell Cycle in E. coli E. coli requires ~40 minutes to replicate its DNA and 20 minutes after termination of replication to prepare for division Figure 6.2

Copyright © McGraw-Hill Companies, Inc. Permission required for reproduction or display. 6 Figure 6.3 Beside determination of cell shape, MreB is thought to play a role in chromosome partitioning to daughter cells

7 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 – 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

8 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 –plays a role in Z ring formation which is essential for septation

Copyright © McGraw-Hill Companies, Inc. Permission required for reproduction or display. 9 Figure 6.4

10 The Fts Z Ring and Cell Division

11 The Fts Z Ring and Cell Division

12 Cell Wall Synthesis Streptococcus hemolyticus

13 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? –2 nd, 3 rd, and 4 th round of replication can begin before first round of replication is completed –Progeny cells are “born” with two or more replication forks, replication already in progress.

14 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

Copyright © McGraw-Hill Companies, Inc. Permission required for reproduction or display. 15 lag phase no increase log phase maximal rate of division and population growth stationary phase population growth ceases death phase decline in population size Figure 6.6

16 Lag Phase Cell synthesizing new components –to replenish spent materials –to adapt to new medium or other conditions Varies in length –in some cases can be very short or even absent

17 Exponential Phase Rate of growth is constant Population is most uniform in terms of chemical and physical properties during this phase During log phase, cells exhibit balanced growth –cellular constituents manufactured at constant rates relative to each other

18 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

19 Possible reasons for entry into stationary phase Nutrient limitation Limited oxygen availability Toxic waste accumulation Critical population density reached

20 Starvation responses Morphological changes –endospore formation Decrease in size, protoplast shrinkage, and nucleoid condensation Production of starvation proteins Long-term survival Increased virulence

21 Death Phase Two new alternative hypotheses –Cells are Viable But Not Culturable (VBNC) Cells alive, but dormant Programmed cell death –Fraction of the population genetically programmed to die (commit suicide)

Copyright © McGraw-Hill Companies, Inc. Permission required for reproduction or display. 22 Loss of Viability Figure 6.8

23 Prolonged Decline in Growth bacterial population continually evolves process marked by successive waves of genetically distinct varients natural selection occurs Figure 6.9

24 The Mathematics of Growth Generation (doubling) time –time required for the population to double in size –varies depending on species of microorganism and environmental conditions –range is from 10 minutes to several days for some microorganisms

Copyright © McGraw-Hill Companies, Inc. Permission required for reproduction or display. 25 Table 6.1

Calculation of number of cells, generation times, and growth rates N o = initial population number N t = population at time t n = number of generations at time t g = generation time

27 If in 8 h an exponentially growing cell population increases from 5 × 10 6 cells/ml to 5 × 10 8 cells/ml, calculate g and n.

Copyright © McGraw-Hill Companies, Inc. Permission required for reproduction or display. 28 Figure 6.10

Copyright © McGraw-Hill Companies, Inc. Permission required for reproduction or display. 29 Figure 6.11 generation time could be determined using a semilogarithmic graph at the log phase

Copyright © McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30 Table 6.2