IN THE NAME OF GODIN THE NAME OF GOD

Islamic Azad University Islamic Azad University Falavarjan BranchFalavarjan Branch

School of Biological SciencesSchool of Biological SciencesDepartment of MicrobiologyDepartment of Microbiology

Microbial GrowthMicrobial Growth

By:By:Keivan Beheshti MaalKeivan Beheshti Maal

GrowthGrowth

increase in cellular constituents - may result in:increase in cellular constituents - may result in:– increase in cell numberincrease in cell number

e.g., reproduction by e.g., reproduction by buddingbudding or or binary binary fissionfission

– increase in cell sizeincrease in cell sizee.ge.g., ., coenocyticcoenocytic microorganisms - nuclear microorganisms - nuclear divisions not accompanied by cell divisionsdivisions not accompanied by cell divisions

microbiologists usually study population growth microbiologists usually study population growth rather than growth of individual cellsrather than growth of individual cells

The Procaryotic Cell CycleThe Procaryotic Cell Cycle

cell cyclecell cycle - sequence of events from - sequence of events from formation of new cell through the next cell formation of new cell through the next cell divisiondivision– most bacteria divide by binary fissionmost bacteria divide by binary fission

two pathways function during cycletwo pathways function during cycle– DNA replication and partitionDNA replication and partition– cytokinesis cytokinesis

Figure 6.1

The Cell Cycle in The Cell Cycle in E. coliE. coli

E. coliE. coli requires ~40 minutes to replicate its requires ~40 minutes to replicate its DNA and 20 minutes after termination of DNA and 20 minutes after termination of replication to prepare for divisionreplication to prepare for division

Figure 6.2

Bacterial growth – exponential.Bacterial growth – exponential.

Daughter cells may separate or remain Daughter cells may separate or remain attached in characteristic arrangements of attached in characteristic arrangements of chains, clusters or pairs. chains, clusters or pairs.

Other forms of reproduction include: Other forms of reproduction include: budding, fragmentation, conidia, or budding, fragmentation, conidia, or sporulation.sporulation.

The Growth CurveThe Growth Curveobserved when microorganisms cultivated in observed when microorganisms cultivated in batch culturebatch culture

– culture incubated in a closed vessel with a culture incubated in a closed vessel with a single batch of mediumsingle batch of medium

Exponential - plotted as logarithm of cell number Exponential - plotted as logarithm of cell number versus timeversus time

– Single parent cell gives rise to two progeny Single parent cell gives rise to two progeny cellscells

usually four distinct phasesusually four distinct phases

Figure 6.6

no increase

maximal rate of divisionand population growth

population growth ceases

decline inpopulationsize

Lag PhaseLag Phase

no cell divisionno cell division – acclimatization – acclimatization

cell synthesizing new components cell synthesizing new components – essential enzymes, cofactors, ATP essential enzymes, cofactors, ATP – replenish spent materialsreplenish spent materials– adapt to new medium or other conditionsadapt to new medium or other conditions

varies in lengthvaries in length– older cells and stressed cells - longer to recover older cells and stressed cells - longer to recover – can be very short or even absentcan be very short or even absent– dependent on bacteria and environmental conditionsdependent on bacteria and environmental conditions

Exponential PhaseExponential Phase

log phase – maximal growthlog phase – maximal growth

rate of growth is constant - rate of growth is constant - steady increasesteady increase

population - most uniform in terms of population - most uniform in terms of chemical and physical properties during chemical and physical properties during this phasethis phase

Maximal rate of exponential growth via binary Maximal rate of exponential growth via binary fissionfission

– metabolic activity peaksmetabolic activity peaks

Generation timeGeneration time = rate of bacterial reproduction = rate of bacterial reproduction– time taken by one individual bacterium to dividetime taken by one individual bacterium to divide– varies according to type of bacterium and varies according to type of bacterium and

environmental conditionsenvironmental conditions

maximum cell concentration dependent on maximum cell concentration dependent on organism and environmentorganism and environment

up to 10up to 101111 bacterial cells per ml bacterial cells per ml

Figure 6.3

each individualcell divides at aslightly differenttime

curve risessmoothly ratherthan as discretesteps

Balanced growthBalanced growth

during log phase, cells exhibit during log phase, cells exhibit balanced growthbalanced growth

– cellular constituents manufactured at constant cellular constituents manufactured at constant rates relative to each otherrates relative to each other

Unbalanced growthUnbalanced growthrates of synthesis of cell components vary rates of synthesis of cell components vary relative to each otherrelative to each other

occurs under a variety of conditionsoccurs under a variety of conditions

– change in nutrient levelschange in nutrient levelsshift-upshift-up (poor medium to rich medium) (poor medium to rich medium)shift-downshift-down (rich medium to poor medium) (rich medium to poor medium)

– change in environmental conditionschange in environmental conditions

Effect of nutrient concentration on Effect of nutrient concentration on growthgrowth

Figure 6.7

Stationary PhaseStationary Phase

Growth/cell division ceasesGrowth/cell division ceases – plateau reached – plateau reached

total number of viable cells remains constanttotal number of viable cells remains constant

– metabolically active cells stop reproducingmetabolically active cells stop reproducing

– reproductive rate is balanced by death ratereproductive rate is balanced by death rate

Possible reasons for entry into Possible reasons for entry into stationary phasestationary phase

nutrient limitationnutrient limitation

limited oxygen availabilitylimited oxygen availability

toxic waste accumulationtoxic waste accumulation

critical population density reachedcritical population density reached

Starvation responsesStarvation responses

morphological changesmorphological changes– e.g., endospore formatione.g., endospore formation

decrease in size, protoplast shrinkage, and decrease in size, protoplast shrinkage, and nucleoid condensationnucleoid condensation

production of starvation proteinsproduction of starvation proteins

long-term survivallong-term survival

increased virulenceincreased virulence

Death PhaseDeath Phase Unfavourable environmental conditions, Unfavourable environmental conditions,

starvation, stressstarvation, stress

cells dyingcells dying, usually at exponential rate, usually at exponential rate

deathdeath– irreversible loss of ability to reproduceirreversible loss of ability to reproduce

in some cases, death rate slows due to in some cases, death rate slows due to accumulation of resistant cellsaccumulation of resistant cells

Viable but non-culturable bacteria – Viable but non-culturable bacteria – VBNCVBNC– Temporarily unable to grow - dormantTemporarily unable to grow - dormant– Can resume growth – environment favourableCan resume growth – environment favourable– Programmed cell survivalProgrammed cell survival

Programmed cell deathProgrammed cell death– Programmed cell suicide by fraction of Programmed cell suicide by fraction of

populationpopulation– Dead cells provide nutrientsDead cells provide nutrients

Loss of ViabilityLoss of Viability

Figure 6.8

The Mathematics of GrowthThe Mathematics of Growth

generation (doubling) timegeneration (doubling) time– time required for the population to double in sizetime required for the population to double in size

– e.g.,2 cells after 20 min; 4 cells after 40 min, etce.g.,2 cells after 20 min; 4 cells after 40 min, etc

– increase in population = increase in population = 22nn; n = no. of generation; n = no. of generation

mean growth rate constantmean growth rate constant– number of generations per unit timenumber of generations per unit time

– usually expressed as generations per hourusually expressed as generations per hour

cells are dividing and doubling in number at regular intervals

Figure 6.10

each individualcell divides at aslightly differenttime

curve risessmoothly ratherthan as discretesteps

CALCULATING THE GROWTH CALCULATING THE GROWTH RATERATE

NN00 = initial population number = initial population number

NNtt = population at time = population at time tt

nn = number of generations in time = number of generations in time tt22nn = generation time = generation time

NNt = t = NN0 0 × 2× 2nn

Which converts down to…Which converts down to…n = (log N - log N0)/0.301n = (log N - log N0)/0.301

Yes…you really should learn this equation…Yes…you really should learn this equation…

To calculate To calculate nn (number of generations): (number of generations):

Log NLog Ntt = log N = log N0 0 + + nn . log 2 . log 2

nn = log N= log Ntt – log N – log Ntt

log 2log 2

= log N= log Ntt – log N – log Ntt

0.3010.301

mean growth rate constant (mean growth rate constant (kk))– number of generations per unit timenumber of generations per unit time

– usually expressed as usually expressed as generations per hourgenerations per hour

– kk == nn / / tt

== log Nlog Ntt – log N – log Noo

0.3010.301tt

Mean generation time (Mean generation time (gg))– If the population doubles (If the population doubles (t = gt = g), then), then

– NNtt = 2N = 2N00

– kk == log (2Nlog (2N00) – log N) – log N00

0.3010.301gg

== log 2 + log log 2 + log NN00 – log N – log N00

0.3010.301gg

kk == 1/1/ggg g = = 1/k1/k

Figure 6.11

Table 6.2Table 6.2

How many cells of How many cells of Staphylococcus aureusStaphylococcus aureus (N (Ntt) will be ) will be present in an egg salad sandwich after it sits in a warm car present in an egg salad sandwich after it sits in a warm car for 4 h?for 4 h?– The number of cells present when the sandwich was The number of cells present when the sandwich was

being prepared was 10 (Nbeing prepared was 10 (N00))– Generation time = 20 minGeneration time = 20 min

NNt t == NN0 0 × 2× 2nn

nn = = t/g = t/g = 240/20 = 12240/20 = 12

22n n = 2= 21212

NNt t == NN0 0 × 2× 2n n = 10 × 2= 10 × 21212

== 10 × 409610 × 4096= 40 960 cells= 40 960 cells

Measurement of Measurement of Microbial GrowthMicrobial Growth

can measure changes in number of cells can measure changes in number of cells in a populationin a population

can measure changes in mass of can measure changes in mass of populationpopulation

Measurement of Cell NumbersMeasurement of Cell Numbers

Direct cell countsDirect cell counts– counting chamberscounting chambers– electronic counterselectronic counters– on membrane filterson membrane filters

Viable cell countsViable cell counts– plating methodsplating methods– membrane filtration methodsmembrane filtration methods

Counting chambersCounting chambers

easy, inexpensive, easy, inexpensive, and quickand quick

useful for counting useful for counting both eucaryotes both eucaryotes and procaryotesand procaryotes

cannot distinguish cannot distinguish living from dead living from dead cellscells

Figure 6.12

Electronic countersElectronic counters

microbial suspension forced through small microbial suspension forced through small orificeorifice

movement of microbe through orifice movement of microbe through orifice impacts electric current flowing through impacts electric current flowing through orificeorifice

instances of disruption of current are instances of disruption of current are countedcounted

Electronic Electronic counterscounters

cannot distinguish living from dead cellscannot distinguish living from dead cells

quick and easy to usequick and easy to use

useful for large microorganisms and blood useful for large microorganisms and blood cells, but not procaryotescells, but not procaryotes

Direct counts on membrane filtersDirect counts on membrane filters

cells filtered through special membrane that cells filtered through special membrane that provides dark background for observing cellsprovides dark background for observing cells

cells stained with fluorescent dyescells stained with fluorescent dyes

useful for counting bacteriauseful for counting bacteria

with certain dyes, can distinguish living from with certain dyes, can distinguish living from dead cellsdead cells

Viable counting methodsViable counting methodsmeasure number of measure number of viable cellsviable cells

Viable – alive and Viable – alive and reproducingreproducing

population size is population size is expressed as expressed as colony colony forming units (CFU)forming units (CFU)

Spread plate Spread plate andand pour plate pour plate methodsmethods

plate dilutions of plate dilutions of population on suitable population on suitable

solid mediumsolid medium

count number of coloniescount number of colonies

calculate number of cells in calculate number of cells in population (cfu)population (cfu)

= = no. of colonies x dilution no. of colonies x dilution factorfactor

simple and sensitivesimple and sensitive

Number calculated from Number calculated from cfucfu and and sample dilutionsample dilution

1 ml of 101 ml of 10-6-6 dilution = 150 cfu dilution = 150 cfuTherefore, original sample had 1.5 Therefore, original sample had 1.5 × 10× 1088 cells cells

widely used for viable counts of microorganisms in widely used for viable counts of microorganisms in food, water, and soilfood, water, and soil

inaccurate results obtained if cells clump togetherinaccurate results obtained if cells clump together

30 -300 colonies 30 -300 colonies

Membrane filtration methodsMembrane filtration methods

Figure 6.13especially useful for analyzing aquatic samples

Fig. 6.14Fig. 6.14

Measurement of Cell MassMeasurement of Cell Mass

dry weightdry weight– time consuming and not very sensitivetime consuming and not very sensitive– Filamentous fungiFilamentous fungi

quantity of a particular cell constituentquantity of a particular cell constituent– e.g., protein, DNA, ATP, or chlorophylle.g., protein, DNA, ATP, or chlorophyll– useful if amount of substance in each cell is useful if amount of substance in each cell is

constantconstant

turbidometric turbidometric – light scattering directly proportional to light scattering directly proportional to

biomass and indirectly proportional to cell biomass and indirectly proportional to cell numbernumber

– spectrophotometryspectrophotometry– quick, easy, and sensitivequick, easy, and sensitive– Cloudiness or Cloudiness or turbidityturbidity of broth of broth

Figure 6.13

more cellsmore lightscatteredless lightdetected

Continuous Culture of Continuous Culture of MicroorganismsMicroorganisms

growth in an open systemgrowth in an open system– continual provision of nutrientscontinual provision of nutrients– continual removal of wastescontinual removal of wastes

maintains cells in log phase at a constant maintains cells in log phase at a constant biomass concentration for extended biomass concentration for extended periodsperiods

continuous culture systemcontinuous culture system

The ChemostatThe Chemostat

rate of incoming rate of incoming medium = rate of medium = rate of removal of removal of medium from medium from vesselvessel

an essential an essential nutrient is in nutrient is in limiting quantitieslimiting quantities

Figure 6.16

Dilution rate and microbial Dilution rate and microbial growthgrowth

Figure 6.17

dilution rate – rate atwhich medium flowsthrough vesselrelative to vessel size

note: cell densitymaintained at widerange of dilutionrates and chemostat operates best at low dilution rate

Population density and generation time linked to Population density and generation time linked to dilution ratedilution rate

Population density unchanged over wide dilution Population density unchanged over wide dilution rate rangerate range

Generation time decreases as dilution rate Generation time decreases as dilution rate increasesincreases– Growth rate increasesGrowth rate increases

Too high dilution rate – washout Too high dilution rate – washout – > maximal growth rate> maximal growth rate

Too low dilution rateToo low dilution rate– Increased cell density and growth rateIncreased cell density and growth rate– Limited nutrient supply availableLimited nutrient supply available

The TurbidostatThe Turbidostatregulates the flow rate of media through vessel to regulates the flow rate of media through vessel to maintain a predetermined turbidity or cell densitymaintain a predetermined turbidity or cell density

photocellphotocell

dilution rate varies – not constantdilution rate varies – not constant

no limiting nutrient – all nutrients in excessno limiting nutrient – all nutrients in excess

turbidostat operates best at high dilution ratesturbidostat operates best at high dilution rates

Importance of continuous culture Importance of continuous culture methodsmethods

constant supply of cells in exponential phase constant supply of cells in exponential phase growing at a known rategrowing at a known rate

study of microbial growth at very low nutrient study of microbial growth at very low nutrient concentrations, close to those present in natural concentrations, close to those present in natural environmentenvironment

study of interactions of microbes under conditions study of interactions of microbes under conditions resembling those in aquatic environmentsresembling those in aquatic environments

food and industrial microbiologyfood and industrial microbiology

Influence of Environmental Influence of Environmental FactorsFactors

Physical and chemical factors required for Physical and chemical factors required for growthgrowth– light, temperature, ph, and osmotic pressurelight, temperature, ph, and osmotic pressure

most organisms grow in fairly moderate most organisms grow in fairly moderate environmental conditionsenvironmental conditions

extremophilesextremophiles– grow under harsh conditions that would kill grow under harsh conditions that would kill

most other organismsmost other organisms

Solutes and Water ActivitySolutes and Water Activity

water activity (awater activity (aww))

– amount of water available to organismsamount of water available to organisms

– reduced by interaction with solute molecules reduced by interaction with solute molecules (osmotic effect)(osmotic effect)

higher [solute] higher [solute] lower a lower aww

– reduced by adsorption to surfaces (matric reduced by adsorption to surfaces (matric effect)effect)

AAww of 0.9 – 1.0 required for microbial growth of 0.9 – 1.0 required for microbial growth

– Fungi grow at lower AFungi grow at lower Aww than bacteria than bacteria

implicated in spoilage of dry foods such as implicated in spoilage of dry foods such as breadbread

– HalotolerantHalotolerant

– osmotolerantosmotolerant

Osmotolerant organismsOsmotolerant organismsgrow over wide ranges of water activitygrow over wide ranges of water activity

OsmophilesOsmophiles – high osmotic pressure for growth – high osmotic pressure for growth– approx. 0.98 - spoilage of sweet foodapprox. 0.98 - spoilage of sweet food

use use compatible solutescompatible solutes to increase their internal to increase their internal osmotic concentrationosmotic concentration– solutes - compatible with metabolism and solutes - compatible with metabolism and

growthgrowth

proteins and membranes that require high solute proteins and membranes that require high solute concentrations for stability and activityconcentrations for stability and activity

Effects of NaCl on microbial growthEffects of NaCl on microbial growth

halophileshalophiles– grow optimally at >0.2 grow optimally at >0.2

MM

extreme halophilesextreme halophiles– require >2 Mrequire >2 M

Figure 6.18

HalophilesHalophilesAdapted to Adapted to saline environmentssaline environments– Some Archaea require 20 – 30% NaClSome Archaea require 20 – 30% NaCl

– HalobacteriumHalobacterium spp spp. from Dead Sea – 6.2 M . from Dead Sea – 6.2 M NaCl (29%)NaCl (29%)

Identify cell ultrastructure adaptations of Identify cell ultrastructure adaptations of halophiles!!halophiles!!

pHpH

negative negative logarithm of logarithm of the hydrogen the hydrogen ion ion concentrationconcentration

acidophilesacidophiles– growth optimum between growth optimum between pH 0 - 5.5pH 0 - 5.5

neutrophilesneutrophiles– growth optimum between growth optimum between pH 5.5 - 7pH 5.5 - 7

alkalophilesalkalophiles– growth optimum between growth optimum between pH 8.5 - 11.5pH 8.5 - 11.5

– Most bacteria and protozoa Most bacteria and protozoa –– neutrophilesneutrophiles

– Most fungi = pH 4-6 Most fungi = pH 4-6 –– acidophilesacidophiles

most acidophiles and alkalophiles maintain an internal pH most acidophiles and alkalophiles maintain an internal pH near neutralitynear neutrality– some use proton/ion exchange mechanisms to do sosome use proton/ion exchange mechanisms to do so

some synthesize proteins that provide protectionsome synthesize proteins that provide protection– e.g., acid-shock proteinse.g., acid-shock proteins

many microorganisms change pH of their habitat by many microorganisms change pH of their habitat by producing acidic or basic waste productsproducing acidic or basic waste products– most media - buffers to prevent growth inhibitionmost media - buffers to prevent growth inhibition

TemperatureTemperature

organisms organisms exhibit distinct exhibit distinct cardinal growth cardinal growth temperaturestemperatures– minimalminimal– maximalmaximal– optimaloptimal

Figure 6.20

Figure 6.21

Temperature OptimaTemperature OptimaPsychrophilesPsychrophiles– 0 – 20 °C – optimum 15 °C0 – 20 °C – optimum 15 °C

PsychrotrophsPsychrotrophs– Prefer 20 – 30 °C, grow at wide range of 0 – 35 Prefer 20 – 30 °C, grow at wide range of 0 – 35

°C°Cspoil refrigerated foodsspoil refrigerated foods

MesophilesMesophiles– 20 – 45 °C 20 – 45 °C – human pathogenshuman pathogens

ThermophilesThermophiles– 55 – 65 °C optimal temperature 55 – 65 °C optimal temperature – Can survive 45 – 100 °C Can survive 45 – 100 °C – compost, hot water springs, deep sea compost, hot water springs, deep sea

volcanoes, rifts, and ridgesvolcanoes, rifts, and ridges

HyperthermophilesHyperthermophiles– 80 -115 °C80 -115 °C

Table 6.5Table 6.5

Adaptations of thermophilesAdaptations of thermophiles

protein structure stabilized by: protein structure stabilized by: – e.g., more H bondse.g., more H bonds– e.g., more prolinee.g., more proline– e.g., chaperonese.g., chaperones

histone-like proteins stabilize DNAhistone-like proteins stabilize DNA

membrane stabilized by:membrane stabilized by:– e.g., more saturated, more branched and higher e.g., more saturated, more branched and higher

molecular weight lipidsmolecular weight lipids– e.g., ether linkages (archaeal membranes)e.g., ether linkages (archaeal membranes)

Oxygen RequirementsOxygen RequirementsAerobesAerobes = require = require atmospheric oxygen (20%) atmospheric oxygen (20%)– Obligate aerobesObligate aerobes

completely dependent on Ocompletely dependent on O22

– Facultative anaerobesFacultative anaerobesOO22 not required but contributes to better growth not required but contributes to better growth

– AerotolerantAerotolerantnot bothered by presence or absence of Onot bothered by presence or absence of O22

– MicroaerophilicMicroaerophilicrequire 2 – 10% Orequire 2 – 10% O22 (lactic acid bacteria) (lactic acid bacteria)

– Yeasts – facultative anaerobesYeasts – facultative anaerobes– Mold/fungi – aerobicMold/fungi – aerobic

Oxygen ConcentrationOxygen Concentration

Figure 6.15

needoxygen

preferoxygen

ignoreoxygen

oxygen istoxic

< 2 – 10%oxygen

Basis of different oxygen Basis of different oxygen sensitivitiessensitivities

oxygen easily reduced to toxic productsoxygen easily reduced to toxic products– superoxide radical superoxide radical – hydrogen peroxidehydrogen peroxide– hydroxyl radicalhydroxyl radical

aerobes produce protective enzymesaerobes produce protective enzymes– superoxide dismutase (SOD)superoxide dismutase (SOD)– catalasecatalase

Oxygen Requirement

AnaerobesAnaerobes– Unable to grow in presence of oxygenUnable to grow in presence of oxygen

– Obligate anaerobesObligate anaerobes – do not tolerate oxygen – do not tolerate oxygen

– Grown in special anaerobic flasks or cabinets in Grown in special anaerobic flasks or cabinets in presence of COpresence of CO22 and N and N22 gas mixtures gas mixtures

– Oxygen toxic to Oxygen toxic to Bacteroides, Clostridium, Bacteroides, Clostridium, Fusobacterium, MethanococcusFusobacterium, Methanococcus

Figure 6.24

PressurePressure

barotolerant organismsbarotolerant organisms– adversely affected by increased pressure, but adversely affected by increased pressure, but

not as severely as nontolerant organismsnot as severely as nontolerant organisms

barophilic organismsbarophilic organisms– require or grow more rapidly in the presence require or grow more rapidly in the presence

of increased pressureof increased pressure

RadiationRadiation

Figure 6.25

Radiation damageRadiation damage

ionizing radiationionizing radiation– x rays and gamma raysx rays and gamma rays

– mutations mutations death death

– disrupts chemical structure of many disrupts chemical structure of many molecules, including DNAmolecules, including DNA

damage repaired by DNA repair damage repaired by DNA repair mechanismsmechanisms

Radiation damage…Radiation damage…ultraviolet (UV) radiationultraviolet (UV) radiation– mutations mutations death death

– causes formation of causes formation of thymine dimersthymine dimers in DNA in DNA

– DNA damage can be repaired by two mechanismsDNA damage can be repaired by two mechanisms

photoreactivationphotoreactivation – dimers split in presence of – dimers split in presence of lightlight

dark reactivationdark reactivation – dimers excised and – dimers excised and replaced in absence of lightreplaced in absence of light

Radiation damage…Radiation damage…

visible lightvisible light– at high intensities generates at high intensities generates singlet oxygensinglet oxygen

((11OO22))

powerful oxidizing agentpowerful oxidizing agent

– carotenoidcarotenoid pigments pigmentsprotect many light-exposed microorganisms from protect many light-exposed microorganisms from photooxidationphotooxidation

Microbial Growth in Natural Microbial Growth in Natural EnvironmentsEnvironments

microbial environments are complex, microbial environments are complex, constantly changingconstantly changing

microorganism exposed to overlapping microorganism exposed to overlapping gradients of nutrients and environmental gradients of nutrients and environmental factorsfactors

often contain low nutrient concentrations often contain low nutrient concentrations ((oligotrophic environmentoligotrophic environment))

Growth Limitation by Environmental Growth Limitation by Environmental FactorsFactors

Leibig’s law of the minimumLeibig’s law of the minimum– total biomass of organism determined by total biomass of organism determined by

nutrient present at lowest concentrationnutrient present at lowest concentration

Shelford’s law of toleranceShelford’s law of tolerance– above or below certain environmental limits, a above or below certain environmental limits, a

microorganism will not grow, regardless of the microorganism will not grow, regardless of the nutrient supplynutrient supply

Responses to low nutrient levelsResponses to low nutrient levels

oligotrophicoligotrophic environments environments

organisms become more competitive in organisms become more competitive in nutrient capture and use of available resourcesnutrient capture and use of available resources

morphological changes to increase surface morphological changes to increase surface area and ability to absorb nutrientsarea and ability to absorb nutrients

mechanisms to sequester certain nutrientsmechanisms to sequester certain nutrients

Counting Viable but Nonculturable Counting Viable but Nonculturable Vegetative ProcaryotesVegetative Procaryotes

stressed microorganisms - temporarily lose stressed microorganisms - temporarily lose ability to grow using normal cultivation methodsability to grow using normal cultivation methods

microscopic and isotopic methods for counting microscopic and isotopic methods for counting viable but nonculturable cells have been viable but nonculturable cells have been developeddeveloped

Quorum Sensing and Quorum Sensing and Microbial PopulationsMicrobial Populations

quorum sensingquorum sensing– microbial communication and cooperationmicrobial communication and cooperation

– involves secretion and detection of chemical involves secretion and detection of chemical signalssignals

concentration present allows cells to access concentration present allows cells to access population densitypopulation density

Quorum SensingQuorum Sensing

acylhomoserine lactone (AHL)acylhomoserine lactone (AHL) - autoinducer - autoinducer molecule produced by many Gram-negative molecule produced by many Gram-negative organisms organisms

AHL or other signal molecule diffuses across AHL or other signal molecule diffuses across plasma membrane plasma membrane

at high concentrations it enters cellat high concentrations it enters cell

once inside the cell it induces expression of once inside the cell it induces expression of target genes that regulate a variety of functionstarget genes that regulate a variety of functions

Figure 6.29

Processes sensitive to quorum Processes sensitive to quorum sensing: gram-negative bacteriasensing: gram-negative bacteria

bioluminescencebioluminescence ( (Vibrio fischeriVibrio fischeri))

synthesis and release of synthesis and release of virulence factorsvirulence factors ((Pseudomonas aeruginosaPseudomonas aeruginosa))

conjugationconjugation ((Agrobacterium tumefaciensAgrobacterium tumefaciens))

antibiotic productionantibiotic production ( (Erwinia carotovora, Erwinia carotovora, Pseudomonas aureofaciensPseudomonas aureofaciens))

biofilm productionbiofilm production ( (P. aeruginosaP. aeruginosa))

Quorum sensing: gram-positive Quorum sensing: gram-positive bacteriabacteria

often mediated by oligopeptide pheromoneoften mediated by oligopeptide pheromone

processes impacted by quorum sensing:processes impacted by quorum sensing:– matingmating ( (Enterococcus faecalisEnterococcus faecalis))– transformation competencetransformation competence ( (Streptococcus Streptococcus

pneumoniaepneumoniae))– sporulationsporulation ( (Bacillus subtilisBacillus subtilis))– production of production of virulence factorsvirulence factors ( (Staphylococcus Staphylococcus

aureusaureus))– development of development of aerial myceliaaerial mycelia ( (Streptomyces griseusStreptomyces griseus))– antibiotic productionantibiotic production ( (S. griseusS. griseus))