Microbial Growth and Physical Requirements

Microbial Growth

• Most bacteria increase in size and divide into two cells
• Binary fission requires the cell to elongate DNA replication
• The cell wall and plasma membrane start to constrict
• A cross-wall forms, separating the two DNA copies
• The cells separate as microbial growth increases cell number, not cell size

Physical Requirements for Growth

• Each species has its own temperative growth response
• Minimum growth temperature
• Optimum growth temperature facilitates the fastest growth
• Maximum growth temperature

Temperature Response

• Psychrotrophs/philes optimum temperature is 25 degrees celcius/10 degrees celcius, grows between 0-30 degrees celcius
• Mesophiles’s optimum growth temperature is 35-37 degrees celcius, most pathogens are mesophiles
• Thermophiles optimum growth temperature is 60 degree celcius (hyper is 90-100 degrees celcius), can grow at temperatures that would kill most mesophiles and are rarely pathogenic

pH

• Bacterial growth occurs between a pH of 5-9
• Most bacteria grow best between pH of 6.5 and 7.5
• Molds and yeasts grow between pH 5 and 6
• Acidiophiles grow in acidic environments and have a pH of 4-5

Osmotic Pressure

• This is measured as water activity or Aw
• Hypertonic environments such as high salt or sugar causes plasmolysis
• Extreme/obligate halophiles require high osmotic pressure
• Facultative halophiles can tolerate high osmotic pressure

Chemical Requirements for Growth

• Carbon is a structural organic molecule and energy source
• Chemoheterotrophs use organic carbon sources
• Autotrophs use CO2 as a sole carbon source, CO2 fixation
• Nitrogen is found in amino acids and proteins
• Most bacteria decompose proteins and some bacteria use NH4+ or NO3
• Nitrogen fixation requires using N2
• Sulfur is found in amino acids, thiamine, B1 and biotin
• Bacteria can decompose proteins and use SO42- or H2S
• Phosphorous is in DNA, RNA, ATP, and membranes
• Inorganic PO43- is a source of phosphorus
• Organic growth factors are organic compounds obtained from the environment, such as vitamins, amino acids, co-factors, purines, and pyrimidines
• Trace elements are enzyme cofactors such as CO, Mn, Zn, CU

Growth of Various Types of Bacteria According to Oxygen

• Obligate aerobes require only aerobic growth and oxygen

• These have Catalase and superoxide dismutase [SOD] which allows toxic forms of oxygen to be neutralized.

• Facultative anaerobes grow both ways, with greater growth in presence of oxygen

• These have catalase and SOD, allowing toxic forms of oxygen to be neutralized.

• Obligate anaerobes is anaerobic growth and growth ceases in presence of oxygen

• These lack enzymes to neutralize harmful forms of oxygen and cannot tolerate oxygen

• Aerotolerant anaerobes have only anaerobic growth, but growth continues in presence of oxygen

• These have only one enzyme SOD which allow harmful forms of oxygen to be partially neutralized.

• Microaerophiles require only aerobic growth with low concentration of oxygen

Toxic Forms of Oxygen

• Singlet oxygen has O2, boosted to a higher-energy stated
• Superoxide free radicals have O2-
• An example is, O2¯ + 2 H+ -->H2O2 + O2
• Peroxide anion is OH22-(H2O2)
• An example is, 2 H2O2--> 2 H2O + O2 or H2O2 + 2 O+ --> 2H₂O
• Detoxifying enzymes are necessary for aerobic growth, these require Superoxide dismutase, catalase, peroxidase

Reproduction in Prokaryotes

• Binary fission, splitting in 2
• Most of microbial species do binary fission
• Budding, conidiospores and fragmentation are also ways population numbers can be increased

Population of Lab VS Natural Growth

• Colony of Broth growth is a pure culture, mass of cells
• It is on culture media but the growth isnt the same as in a natural environment
• Biofilm is a natural environment with a thick layer of 10-200 um
• Biofilm is complex with unique interactions, and creates impact on microorganisms

Microbial Communities

• Form slime or hydrogels
• Bacteria are attracted to biofilm by chemicals

Quorum sensing

• This process means cells produce and detect unique chemicals
• Cells can sense when other cells are present (and how many)
• The cells can change behavior

Growth Equations

• N = Nox 2n
• N0 is the starting # of cells
• Nt = final population
• n = number of generations
• 100 or 100 x 64 = 6400 cells in 3 hours
• N₁ (final number of cells)= No (starting # of cells) X 2η
• n (number of generations) =144 (3/hr x 48 hr)
• E. coli mass of cells = N₁=2.2X1043 cells
• One E. coli cell = ~10-12 gram
• X # of cells = 2.2 X 1031 grams of E. coli
• = 24 X 1024 tons
• 4000x the mass of the Earth

Phases of Growth

• Lag Phase is the adjustment to new conditions, rapid matabolism
• Requires ribosome synthesis and no cell division
• Exponential Phase is logarithmic growth, maximum rate of division
• It's most sensitive to antimicrobial agents, including antibiotics
• Stationary Phase means that key nutrients are running out and waste products are building up
• The number of cell deaths is equal to the number of cell divisions
• Death Phase means that cell deaths exceed cell divisions
• Many cultures die exponentially

Microbial Growth Measurement

• Plate count using serial dilutions of a sample
• Perform serial dilutions to dilute the previous dilution

Viable Plate Count

• This is when Petri plates from serial dilutions can be inoculated at 0.1ml
• They are then incubated and colonies counted
• After incubation, count colonies on plates that have 30-300 colonies (CFUs)

Measurement Techniques

• Membrane Filtration requires filtering liquids through a membrane filter and placing filters on mediums and then incubating and growing colonies
• Microscopic count requires adding bacterial suspension there and fill the shallow volume over the squares by capillary action
• All cells in several large squares counted, then the numbers are averaged
• Turbidity measured in Spectrophotometer requires bacterial suspension and is measured at it's light source