Chapter 6

Many definitions for the exam.

Chapter 6
Microbial Nutrition and Growth
Metabolism Results in Reproduction
Microbial growth – an increase in a population of microbes rather than an increase in size

Discete Colonies: cells arising from single parent cell

Reproduction = growth
Sources of Carbon, Energy, Electrons
Organisms categorized into groups
Autotrophs: use an inorganic carbon source (carbon dioxide) are autotrophs
Heterotrophs: catabolize reduced organic molecules (proteins, carbohydrates, amino acids, and fatty acids) are heterotrophs
Chemotrophs: acquire energy from redox reactions involving inorganic and organic chemicals are chemotrophs
Phototrophs: use light as their energy source are phototrophs

Bacterial Cell Division  Cell Growth and Binary Fission
Microbial Growth
Microbial growth involves an increase in the number of cells.
Growth of most microorganisms occurs by the process of binary fission


FTS Proteins
Fts proteins regulate cell division and chromosome replication

Fts proteins interact to form a division apparatus in the cell called the divisome.

The protein FtsZ defines the division plane in prokaryotes

Mre proteins help define cell shape

Peptidoglycan Synthesis and Cell Division
New cell wall is synthesized by inserting new glycan units into preexisting wall material
Cell lysis called autolysis can occur unless new cell wall precursors are spliced into existing peptidoglycan
Bactoprenol facilitates transport of new glycan units through the cytoplasmic membrane to become part of cell wall
Transpeptidation bonds the precursors into the peptidoglycan fabric

Growth of Bacterial Populations 

Exponential Growth: Microbial populations show a characteristic type of growth pattern which is best seen by plotting the number of cells over time on a semilogarithmic graph

The Growth Cycle
Microorganisms show a characteristic growth pattern (Figure 6.8) when inoculated into a fresh culture medium.

Lag, Stationary, Death Phases
lag phase, then exponential growth commences. As essential nutrients are depleted or toxic products build up, growth ceases, and the population enters the stationary phase. If incubation continues, cells may begin to die (the death phase).
  Direct Measurements of Microbial Growth: Total and Viable Counts

Growth is measured by the change in the number of cells over time. Cell counts done microscopically measure the total number of cells in a population,
viable cell counts (plate counts) measure only the living, reproducing population.

Problem With Plate Counts
"the great plate count anomaly"
occurs because microscopic methods count dead cells whereas viable methods do not
different organisms may have different requirements for growth conditions in culture.
Membrane Filtration

Indirect Measurement of Growth
Turbidity measurements are an indirect but rapid method of measuring growth.

To relate a direct cell count to a turbidity value, a standard curve must be established.

Physical Requirements for Growth
Temperature
pH
Osmolarity
Pressure

Growth and Temperature
Psychrophiles: function best at cold temperatures.
Psychrotolerant: Organisms that grow at 0ºC but have optima of 20ºC to 40ºC are called Mesophiles
Thermophiles: Organisms with growth temperature optima between 45ºC and 80ºC. a
Hyperthermophiles. Organims with growth temperature optima greater than 80°C are called

Microbial Growth and pH
Most organisms grow best between pH 6 and 8.
Acidophiles: Organisms that grow best at low pH
Alkaliphiles: Organisms that grow best at high pH

Growth in High Salt Concentrations
Halophiles: grow best at reduced water concentration and higher salt concentrations
Extreme halophiles: require high levels of salts for growth.
Halotolerant: organisms that tolerate some reduction in the water activity but generally grow best in the absence of an added solute, like salt.

Oxygen and Microbial Growth
Aerobes: require oxygen
Anaerobes: do not require oxygen
Facultative organisms: can live with or without oxygen.
Aerotolerant anaerobes: can tolerate oxygen and grow in its presence even though they cannot use it.
Microaerophiles: aerobes that can use oxygen only when it is present at levels reduced from that in air.

Culture Requirements
Special techniques are needed to grow aerobic and anaerobic microorganisms

Toxic Forms of Oxygen
Several toxic forms of oxygen can be formed in the cell, but enzymes are present in organisms that can neutralize most of them

Superoxide in particular seems to be a common toxic oxygen species.

Growth Requirements
Organisms use a variety of nutrients for energy and to build organic molecules and cellular structures

Most common nutrients: carbon, oxygen, nitrogen, and hydrogen

Four toxic forms of oxygen
Singlet oxygen – molecular oxygen with electrons boosted to higher energy state
Superoxide radicals – some form during incomplete reduction of oxygen in aerobic and anaerobic respiration
aerobes produce superoxide dismutases
anaerobes lack superoxide dismutase and die
Peroxide anion – formed during reactions catalyzed by superoxide dismutase and other reactions
Aerobes contain either catalase or peroxidase
Obligate anaerobes either lack both enzymes
Hydroxyl radical
(Aerobes also use antioxidants such as vitamins C and E to protect against toxic oxygen products)


Classification of Organisms Basedon Oxygen Requirements
Aerobes –use aerobic respiration
Anaerobes – no aerobic metabolism
Facultative Anaerobes –use either fermentation, anaerobic respiration, aerobic respiration
Aerotolerant anaerobes – no aerobic metabolism; only some enzymes to detoxify oxygen
Microaerophiles – aerobes that require oxygen levels from 2-10% and have problems with hydrogen peroxide and superoxide radicals

pH
Organisms sensitive to acidity because H+ and OH- interfere with H bonding in proteins and nucleic acids
Neutrophiles: Most bacteria and protozoa grow best around neutral pH (6.5-7.5)
Acidophiles: Some bacteria and fungi grow best in acidic habitats
Acidic waste products can help preserve foods by preventing further microbial growth
Alkalinophiles: live in alkaline soils and water up to pH 11.5

Osmotic Pressure
Hypertonic solutions have greater solute concentrations; cells placed in these solutions will undergo plasmolysis (shriveling of cytoplasm)
This effect helps preserve some foods

Restricts organisms to certain environments
Obligate halophiles – grow in up to 30% salt
Facultative halophiles – can tolerate high salt concentrations
Hydrostatic Pressure
Water exerts pressure in proportion to its depth
For every addition of depth, water pressure increases 1 atm

Organisms that live under extreme pressure are barophiles
Their membranes and enzymes depend on this pressure to maintain their three-dimensional, functional shape