L3:Bacterial Growth

Bacterial Identification

• Treponema pallidum is diagnosed by direct microscopy and serology.
• Mycobacterium leprae is found in the footpads of mice and nine-banded armadillos.
• Chlamydia trachomatis only grows within eukaryotic cells.

Microscopy

• Coccobacillus shaped bacteria include Chlamydia.
• Rod shaped bacteria include Bacillus.
• Palisades are a characteristic arrangement of bacteria like Corynebacteria.
• Diplobacillus shaped bacteria include Moraxella.
• Cocci include Diplococci like Pneumococcus, Chains like Streptococci and Bunches of grapes like Staphylococci.
• Comma shaped bacteria include Vibrios.
• Spiral-few turns bacteria include Spirillium.
• Spiral-many turns bacteria include Spirochaetes.

Staining and Microscopy

• Simple stain is usually one colour.
• Negative stain is good for observing capsules.
• Fluorescent stain can be used for visualization.
• Differential stain is a technique for distinguishing between different types of bacteria.

Biochemical Tests

• Biochemical tests analyze sugar fermentation products like acids, alcohol, or gas.
• Tests determine the ability to utilize specific sugars.
• Tests analyze bacterial enzymes.
• Growth under aerobic and anaerobic conditions can be tested.

Other Identification Methods

• API system uses a series of biochemical tests to quickly identify bacterial species.
• Catalase test determines whether an organism produces the enzyme catalase.
• Molecular techniques are increasingly used for bacterial identification.
• Matrix Assisted Laser Desorption Time of Flight (MALDI-TOF) can be used to identify bacteria directly from clinical samples like blood.

Growth of Bacteria

• Bacteria divide by binary fission.
• Growth rate varies greatly between bacterial species.
• Generation time refers to the time it takes for one bacterial cell to divide into two.
• Bacterial growth is logarithmic.
• The formula N = 2n calculates the number of bacteria (N) after a certain number of generations (n).

Growth Curve

• The bacterial growth curve depicts the phases of bacterial growth.
• Lag phase is a period of slow growth where bacteria adapt to the new environment.
• Exponential phase is a period of rapid growth where bacteria divide at their maximum rate (µmax).
• Stationary phase is a period of stable population where the rate of cell division is equal to the rate of cell death.
• Death phase is a period where the rate of cell death exceeds the rate of cell division.

Measuring Bacterial Growth

• Direct methods involve counting bacterial cells.
• Microscopy can be used to visually count bacteria.
• Flow cytometry counts cells passing through a light detector.
• Plate counts quantify the number of viable bacteria in a sample.

Indirect Methods

• Optical density measures the turbidity of a bacterial culture.
• Measuring respiration quantifies the metabolic activity of the bacteria.

Bacterial Growth Media

• Broth is a liquid media used for enrichment and growth curve analysis.
• Agar is a solid media used for bacterial identification and assessing culture purity.
• Sloppy agar is a semi-solid media used to demonstrate bacterial motility and preserve bacterial cultures.

Bacterial Identification Based on Colony Morphology

• Color can be indicative of bacterial species.
• Shape can help distinguish bacteria.
• Size can be a determining factor.
• Smell is sometimes a characteristic feature.
• Effect on media can indicate specific bacterial features like haemolysis or color change.

Non-culturable Bacteria

• Some bacteria, like Treponema pallidum, have never been successfully grown in culture.