Chapter 6

Microbial Growth

© 2013 Pearson Education, Inc. Lectures prepared by Christine L. Case

The Requirements for Growth

• Physical requirements– Temperature– pH– Osmotic pressure

• Chemical requirements– Carbon– Nitrogen, sulfur, and phosphorous– Trace elements– Oxygen– Organic growth factor

Figure 6.1 Typical growth rates of different types of microorganisms in response to temperature.

PsychrophilesPsychrotrophs

Mesophiles

Thermophiles

Hyperthermophiles

Applications of Microbiology 6.1 A white microbial biofilm is visible on this deep-sea hydrothermal vent. Water is being emitted through the ocean floor at temperatures above 100°C.

pH

• Most bacteria grow between pH 6.5 and 7.5• Molds and yeasts grow between pH 5 and 6• Acidophiles grow in acidic environments

Osmotic Pressure

• Hypertonic environments, or an increase in salt or sugar, cause plasmolysis

• Extreme or obligate halophiles require high osmotic pressure

• Facultative halophiles tolerate high osmotic pressure

Figure 6.4 Plasmolysis.

Plasma membraneCell wall

Cytoplasm

H2O

NaCl 10%

Cytoplasm

Plasma membrane

Cell in isotonic solution. Plasmolyzed cell in hypertonic solution.

NaCl 0.85%

Chemical Requirements

• Carbon– Structural organic molecules, energy source– Chemoheterotrophs use organic carbon sources– Autotrophs use CO2

Chemical Requirements

• Nitrogen– In amino acids and proteins– Most bacteria decompose proteins– Some bacteria use NH4

+ or NO3–

– A few bacteria use N2 in nitrogen fixation

Chemical Requirements

• Sulfur– In amino acids, thiamine, and biotin– Most bacteria decompose proteins– Some bacteria use SO4

2– or H2S

• Phosphorus – In DNA, RNA, ATP, and membranes– PO4

3– is a source of phosphorus

Chemical Requirements

• Trace elements– Inorganic elements required in small amounts– Usually as enzyme cofactors

Table 6.1 The Effect of Oxygen on the Growth of Various Types of Bacteria

Organic Growth Factors

• Organic compounds obtained from the environment

• Vitamins, amino acids, purines, and pyrimidines

Biofilms

• Microbial communities• Share nutrients• Sheltered from harmful factors

Figure 6.5 Biofilms.

Clumps of bacteria adhering to surface

Surface Water currents

Migrating clump of bacteria

Applications of Microbiology 3.2 Pseudomonas aeruginosa biofilm.

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Culture Media

• Culture medium: nutrients prepared for microbial growth

• Sterile: no living microbes• Inoculum: introduction of microbes into

medium• Culture: microbes growing in/on culture

medium

Agar

• Complex polysaccharide • Used as solidifying agent for culture media in

Petri plates, slants, and deeps• Generally not metabolized by microbes• Liquefies at 100°C• Solidifies at ~40°C

Culture Media

• Chemically defined media: exact chemical composition is known

• Complex media: extracts and digests of yeasts, meat, or plants– Nutrient broth– Nutrient agar

Table 6.2 A Chemically Defined Medium for Growing a Typical Chemoheterotroph, Such as Escherichia coli

Table 6.4 Composition of Nutrient Agar, a Complex Medium for the Growth of Heterotrophic Bacteria

Anaerobic Culture Methods

• Reducing media– Contain chemicals (thioglycolate or oxyrase) that

combine O2

– Heated to drive off O2

Figure 6.6 A jar for cultivating anaerobic bacteria on Petri plates.

Lid with O-ring gasket

Envelope containing sodium bicarbonate and sodium borohydride

Anaerobic indicator (methylene blue)

Petri plates

Clamp with clamp screw

Palladium catalyst pellets

Figure 6.7 An anaerobic chamber.

Arm ports

Air lock

Capnophiles

• Microbes that require high CO2 conditions

• CO2 packet

• Candle jar

• Make it easy to distinguish colonies of different microbes

Differential Media

Figure 6.9 Blood agar, a differential medium containing red blood cells.

Bacterial colonies

Hemolysis

Figure 6.10 Differential medium.

Uninoculated

Staphylococcusepidermis

Staphylococcusaureus

• Suppress unwanted microbes and encourage desired microbes

Selective Media

Table 6.5 Culture Media

• Binary fission• Budding• Conidiospores (actinomycetes)• Fragmentation of filaments

ANIMATION Bacterial Growth: Overview

Reproduction in Prokaryotes

Figure 6.12a Binary fission in bacteria.

Cell elongates and DNA is replicated.

Cell wall and plasma membrane begin to constrict.

Cross-wall forms, completely separating the two DNA copies.

Cells separate.

Cell wall

Plasma membrane

DNA (nucleoid)

(a) A diagram of the sequence of cell division

Figure 6.12b Binary fission in bacteria.

(b) A thin section of a cell of Bacillus licheniformis starting to divide

Cell wallDNA (nucleoid)

Partially formed cross-wall

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Figure 6.13b Cell division.

Lag PhaseIntense activity preparing for population growth, but no increase in population.

Log PhaseLogarithmic, or exponential, increase in population.

Stationary PhasePeriod of equilibrium; microbial deaths balance production of new cells.

Death PhasePopulation Is decreasing at a logarithmic rate.

The logarithmic growth in the log phase is due to reproduction by binary fission (bacteria) or mitosis (yeast).

Figure 6.15 Understanding the Bacterial Growth Curve.

Staphylococcus spp.