Microbiology Quiz: Biofilms and Cultures

Microbial Growth

• Microbial growth is the increase in population size from cell division.
• Binary fission is the process of cell division where a cell grows to twice its size and then divides into two identical cells.

Cell Nutrition

• Macronutrients are required in large quantities for bacteria.
• Average bacterial cell weight is 10⁻¹² g and 75% is water.
• Carbon, oxygen, nitrogen, hydrogen, phosphorus, and sulfur make up about 96% of dry weight.
• Potassium, sodium, calcium, magnesium, chlorine, and iron make up the remaining 3.7% of dry weight.
• Micronutrients are required in minute amounts.
• The elements needed for microbes are essential to all microorganisms.
• Some trace metals are used for special functions.
• Others are unnecessary but metabolized, and still other are unnecessary and not metabolized.

Growth Media and Laboratory Culture

• Culture medium is a nutrient solution for growing specific microorganisms in a laboratory.
• Defined media has a known chemical composition.
• Complex media has unknown nutrient quantities, often derived from natural sources like yeast extract or peptone.
• Examples of complex media include nutrient agar and tryptic soy broth.
• Defined media is used for genetic studies, and complex media is a source of C, N, vitamins, growth factors, trace elements, and minerals.
• Selective media inhibits the growth of unwanted organisms, while allowing a specific target organism to grow.
• Differential media distinguishes between organisms based on special biochemical reactions.
• Examples are agar for Salmonella and blood agar for hemolysis.
• Enrichment media contains additional nutrients to support the growth of fastidious organisms.

Laboratory Culture

• Liquid media (broths) are used for growing large numbers of microorganisms.
• Solid media contains agar to solidify the medium, used for isolating and observing colonies.
• Semi-solid media contains lower agar concentrations, and is useful for motility studies.
• Colonies are visible clusters of microorganisms that originate from a single microbial cell on solid media.
• Samples inoculated from mixed cultures or contaminated pure cultures typically contain more than one colony type.

Assessing Microbial Numbers

• Microscopic counts enumerate microbial cells.
• Spread plate, pour plate, and optical density are methods for viable counting, where viable cells form colonies.
• Dilution of samples can affect viability.

Turbidimetric Measures of Microbial Cell Numbers

• Turbidity is a measure of cloudiness due to light scattering by microbial cells, where more cells lead to higher turbidity.
• Optical density, or OD, is a measure of turbidity that can be quantitatively assessed about different cell numbers.

Limitations of Microbial Growth Measurements

• Some bacteria form clumps or biofilms, affecting measurements, which may indicate inaccurately how many cells are present.

Dynamics of Microbial Growth

• Growth is the increase in the number of cells through cell division.
• Binary fission is a major process of cell division, where one cell splits into two identical cells.
• Generation time is the time required for a microbial population to double.

Growth Cycle

• Batch culture is a closed-system microbial culture with a fixed volume and limited nutrients.
• Microbial growth profile is called the microbial growth curve.

Phases of the Microbial Growth Curve

• Lag phase is the delay at the beginning of growth, prior to the onset of growth.
• Exponential phase refers to the growth period when the growing cell population doubles at regular intervals.
• Stationary phase is the period when the rate of growth and death are approximately equal.
• Decline phase is the period when the rate of cell death exceeds the rate of cell growth.

Quantitative Aspects of Microbial Growth

• The equation Nt = N₀2^n describes the relationship between cell number at time t (Nt), initial cell number (N₀), and the number of generations (n).

Continuous Culture

• A chemostat is a continuous culture device that allows for precise control over specific growth rate and growth yield of microbial cultures.

Biofilms

• Biofilms are populations of cells embedded in a polysaccharide matrix, attached to a surface.
• Biofilm formation is a process beginning with attachment, colonization, development, and dispersal.
• Biofilms are often associated with chronic infections.

Environmental Effects on Growth: Temperature

• Cardinal temperatures are minimum, maximum, and optimal growth temperatures for a given organism.
• Psychrophiles have an optimal temperature below 15°C, mesophiles between 20°C-45°C, thermophiles greater than 45°C, and hyperthermophiles even higher temperatures.

Molecular Adaptations to Life in the Cold

• Cold-active enzymes possess significant structural alterations, such as more a-helix presence, greater polar and lesser hydrophobic amino acid content, membrane fatty acids with higher unsaturated and shorter chain characteristics to maintain semifluid state and cryopreservants to prevent crystal formation.
• Slime can also be indicative.

Molecular Adaptations to Life at High Temperatures

• Heat-stable enzymes from thermophiles possess special amino acid sequences and solute concentration differences such as di-inositol phosphate, diglycerol phosphate, higher content of long-chain saturated fatty acids, and lipid-layered differences from their mesophilic counterparts.

Effect of pH on Microbial Growth

• Acidophiles thrive in acidic environments and alkaliphiles in alkaline environments.
• Helicobacter pylori uses urease to neutralize stomach acid.
• Alkaliphiles are often from soda lakes or those with high concentrations of carbonate salts.
• Molecular adaptations for alkaliphilic growth include sodium, instead of hydrogen ions, driving transport reactions. These adaptations allow them to maintain a neutral cytoplasm in alkaline pH.

Effects of Osmolarity on Microbial Growth

• Halophiles require NaCl for growth, while halo-tolerant microbes can withstand high salt concentrations.

Effect of Oxygen on Microbial Growth

• Aerobes use oxygen in respiration, whereas anaerobes cannot.
• Microaerophiles require lower oxygen levels, while aerotolerant anaerobes are unaffected by oxygen.
• Obligate anaerobes cannot grow in the presence of oxygen.