Bacterial Growth and Biofilms
Bacterial Replication and Growth
Cytokinesis: Cell Division in Bacteria
Definition: Cytokinesis refers to the division of the cell (
cytomeaning cell,kinesismeaning movement or division).Mechanism: This process in bacteria is facilitated by a contractile ring.
Z-ring (FtsZ ring): This ring is composed of proteins structurally analogous to actin, which in eukaryotic cells is known for muscle contraction.
Pinching Action: The Z-ring forms around the cytoplasm and cell membrane, contracting inwards. This action is similar to tying a string around a balloon and tightening it, causing the balloon to pinch in.
Cell Wall Formation (Septum):
Simultaneous Process: Concurrently with the membrane pinching inwards due to the Z-ring contraction, a new septum (meaning 'wall') is actively formed.
Purpose: While the Z-ring constricts the membrane, the new cell wall septum ensures the complete separation of the two daughter cells, as just membrane pinching would not fully divide the organism if the cell wall remains continuous.
Progression: The membrane pinches in, and more cell wall (septum) forms; this continues until the Z-ring fully contracts and a complete new septum divides the original cell into two distinct bacteria.
Bacterial Chromosome Replication
Nature of Bacterial Chromosome: Bacteria possess a single, circular chromosome made of double-stranded DNA.
Bidirectional Replication: Due to the circular nature and the need for rapid division, bacterial chromosomes replicate
bidirectionally.Process: DNA synthesis starts at a single origin and proceeds in two opposite directions around the circular chromosome.
Efficiency: This simultaneous replication allows the chromosome to be copied in half the time compared to unidirectional replication, facilitating rapid cell division.
Binary Fission
Definition: Binary fission is the primary method of asexual reproduction in most bacteria.
Process: The parental bacterium copies its single chromosome, elongates, and then divides into two genetically identical daughter cells.
Speed: This process can occur very rapidly, with some bacteria replicating every 10 minutes. This leads to exponential growth, where the number of bacteria doubles with each generation.
Example: Starting with 2 bacteria replicating every 30 minutes, after 12 hours (24 replication cycles), the number of bacteria would be 2^{24} (approximately 16.7 million).
Real-world impact: This rapid multiplication explains how infections can quickly lead to an overwhelming number of microbes (e.g., 2^{72} microbes in 24 hours under ideal conditions).
Bacterial Growth Curve in a Closed System
Closed System Definition: A laboratory environment where bacteria are inoculatedb into a fixed amount of nutrients, and no additional nutrients are supplied, nor are waste products removed (e.g., a petri plate or a flask).
Limitations: In a closed system, growth is limited by the depletion of nutrients, accumulation of toxic waste products, and limited space, preventing indefinite exponential growth.
Phases of Growth:
Lag Phase:
Description: An initial period where bacteria adjust to their new environment. There is metabolic activity and growth in cell size, but little to no increase in cell number.
Detectability: The number of bacteria is often too low to be easily detected in samples (e.g., taking a drop from a liter with only 2 or 4 bacteria is unlikely to yield a colony).
Duration: Can last from less than an hour to several days, depending on the species and environmental conditions.
Log Phase (Exponential Growth Phase):
Description: Bacteria are actively undergoing binary fission, doubling their numbers with each generation period. Growth is rapid and exponential.
Ideal State: This is the phase where bacteria are metabolically most active and healthy, making it ideal for studying bacterial properties or for industrial production of bacterial products.
Visualization: On a logarithmic graph, the plot of the number of cells versus generation time is linear.
Stationary Phase:
Description: The rate of bacterial replication equals the rate of bacterial death. The total number of viable cells remains relatively constant.
Causes: Nutrient depletion becomes significant, and waste products accumulate to inhibitory levels.
Implications: Bacteria in this phase may experience stress, leading to a higher rate of mutations. Using fresh cultures from the log phase is generally preferred for consistency in experiments.
Death Phase (Decline Phase):
Description: The rate of bacterial death significantly exceeds the rate of replication. The number of viable bacteria decreases rapidly.
Causes: Severely depleted nutrients and excessive accumulation of toxic waste products make the environment unsustainable for most cells.
Survival: Some cells may switch to a dormant metabolic state to survive, but the overall population declines.
Open System Growth (Chemostat)
Purpose: To maintain bacteria in the log (exponential growth) phase continuously for extended periods, especially in industrial or research settings.
Mechanism: A chemostat enables an open system by continuously supplying fresh nutrients and removing waste products.
Constant Nutrient Supply: Fresh sterile growth medium is continuously dripped into the culture vessel.
Gas Supply: Sterile air or specific gases are continuously bubbled through the culture to ensure adequate oxygenation (or other necessary gases) for metabolic processes like the electron transport chain.
Waste Removal: An overflow tube allows for the continuous removal of spent medium, waste products, and excess bacterial cells, thus preventing toxic buildup and overcrowding.
Benefits: Maintains a consistent environment, ensures high metabolic activity, and stable population size, which is critical for continuous production of substances like antibiotics where mutations or culture decline would be detrimental.
Estimating Bacterial Numbers: Serial Dilution
Problem: When bacterial cultures are very dense (e.g., billions or trillions of cells per milliliter), it's impossible to count individual colonies on a plate.
Solution: Serial dilution is a technique used to systematically reduce the concentration of bacteria in a sample to a countable range.
Process:
Initial Sample: Take 1 milliliter (mL) of the concentrated bacterial culture.
Dilution Steps: Add this 1 mL to a test tube containing 9 mL of sterile diluent (e.g., broth or saline). This creates a 1:10 dilution (or 10^{-1} dilution).
Repeat: From the first diluted tube, take another 1 mL and add it to a new tube with 9 mL of diluent, creating a 1:100 dilution (10^{-2} from the original). This process is repeated serially, typically creating dilutions like 10^{-3}, 10^{-4}, 10^{-5}, 10^{-6}. Note: A fresh pipette is used for each transfer to avoid contamination and ensure accurate dilution.
Plating: Small aliquots (e.g., 1 mL) from several selected dilution tubes are spread onto agar plates and incubated.
Counting: After incubation, plates are examined for colonies (each representing an initial viable bacterial cell).
Target Count: The ideal range for counting colonies on a plate is between 30 and 300. Plates outside this range are generally considered unreliable (too few colonies might not be representative, too many are impossible to count accurately).
Calculation Example:
If a plate from the 10^{-4} dilution yields 32 colonies, the original sample concentration would be calculated as:
(32 ext{ colonies}) imes (10^4 ext{ dilution factor}) = 320,000 ext{ bacteria/mL} or 3.2 imes 10^5 ext{ bacteria/mL} (Colony Forming Units per milliliter, CFU/mL).
Biofilms
Definition: Biofilms are complex communities of one or more types of microorganisms (bacteria, protists, fungi, archaea) that adhere to surfaces and are encased in a self-produced extracellular polymeric substance (EPS) matrix.
Ubiquity: They can form on virtually any surface, organic or inorganic, where moisture is available:
Natural Examples: Dental plaque on teeth, pond scum, slimy layers on rocks in creeks.
Man-made Examples: Metals, minerals, plant tissue, animal tissue, and critically, implanted medical devices (catheters, stents, pacemakers, artificial valves, joint replacements).
Synergistic Relationship: Organisms within a biofilm often exhibit a synergistic relationship, meaning they mutually benefit from living together, performing better as a community than as individual, free-floating (planktonic) cells.
EPS Matrix (Extracellular Polymeric Substance):
Composition: A complex mix of polysaccharides, proteins, nucleic acids, and lipids secreted by the microorganisms.
Function: Acts like a