Phenotypic characteristics and Phenotypic changes
Phenotypes are observable characteristics of an organism. It include any characteristics of a microorganism such as morphological characteristics, cultural characteristics, biochemical characteristics etc.
1. Morphological characteristics:
- It is the overall visual properties of an organism. Such as shape, size and arrangement of bacterial cell, colony morphology, antigenic structure, pilli and flagella etc.
On the basis of shape and arrangements of the cell, bacteria are classified into following types
I) Coccus: each individual cell is spherical or rounded. Coccus shaped bacteria are found as-
Diplococcus- arranged in group of two
Tetracocccus-arranged in group of four
Streptococcus- arranged in chain
Staphylococcus-arranged in cluster
Sarcina- arranged in box shaped in the group of 8, 16 or 32
II) Bacillus: each individual cell is elongated rod shaped. Rod shaped bacteria are arranged as-
single rod (bacillus)
Diplobacillus- arranged in group of two
Streptobacillus- arranged in chain
Palisade- arranged in parallel stack
III) Helical: each individual cell in neither round nor rod, but twisted. Some examples are-
Spiral- cell is spirally twisted
Vibrio- cell is comma shaped
- In the morphology, cell wall of bacteria is very crucial for identification as well as classification of bacteria. Bacteria is broadly classified as Gram Positive or Gram Negative by gram staining technique which is based on difference in their cell wall composition.
2. Cultural characteristics:
- Bacteria are cultured in artificial culture media provided with all the minimal nutrients required for growth and multiplication. After optimum time of incubation in optimum temperature, bacterial colony appears on the culture plate. Most of the bacteria grow in simple nutrient agar media.
The appearance of colony morphology in agar medium have following characteristics.
Size: measured in mm. usually 2-3 mm
Shape: circular, irregular, radiate etc
Surface: smooth, rough, granular, wavy etc
Elevation: flat, convex, elevated.
Edged: entire, lobate, eiliated,
Opacity: opaque, transparent, translucent
Color: pigment production, haemolysis,
Consistency: mucoid, friable etc
- The appearance of bacterial colony on the surface of solid agar media is usually the characteristic of that bacteria. For example; staphylococcus produces round, opaque, yellow colored colony on Nutrient agar.
- Similarly, E. coli produces 2-3 mm round, flat, mucoid, cream colored colony on NA.
- Pin head sized colony is the characteristics of Staphylococci while pin point sized colony is characteristics of Streptococci.
Several factors should be taken into account for artificial culture of bacteria in lab. Some of the major factors are-
i) Optimum temperature for growth: most of the pathogenic bacteria grow at room temperature of 37 C.
ii) Optimum time of incubation for growth: usually 18-48 hours is required for most of the bacteria. Some are slow grower eg. Mycobacterium species requires 2-8 weeks of incubation.
iii) Atmosphere for growth: aerobic, anaerobic or microaerophilic condition should be maintained according to the requirement of the bacteria during culture.
iv) Nutrition requirements: culture media should contains all the basic nutrition for the growth of bacteria. Some bacteria are fastidious, they require additional growth factors in culture media. For example, Haemophilus spp require X and V factor for growth.
3. Biochemical characteristics:
- Different bacteria differ in their metabolic properties.
- Bacterial species having similar morphological and cultural characteristics may show metabolic differences which can determined by biochemical tests.
- Some of the commonly used biochemical tests are O/F test, Methylred test (MR-test), Voges proskaur test (VP-test), Citrate utilization test, Indole production test, urease test, Triple sugar iron test, catalase test, coagulase test, oxidase test etc.
Phenotypic change: Morphological, Cultural and Biochemical modifications
The microorganism have the ability to survive in varying environmental condition and while surviving they show phenotypic changes according to the environment which is known as physiological adaptation.
The phenotypic differences or variation in the organism depends upon the genetic content of the organism and environmental factors. However, variation is also observed in genetically identical organisms under the same environmental conditions.
Genetically uniform bacteria can exhibit heterogeneity (intra-population difference) on some phenotypic characteristics such as difference in metabolism as well as variation in growth rate. Additionally, phenotypic heterogeneity in more complex developmental processes where one sub-population shows varying in phenotypic characteristics than other sub population and this changes in phenotypic characteristics ensure their survival in adverse environmental conditions.
Phenotypic changes is mediated by two mechanisms:
- Genotypic alterations such as, mutations and rearrangements of specific DNA fragments (conjugation, transformation, transduction)
- Epigenetic phenomenon.
Factor influencing phenotypic changes:
- Change in the composition and availability of culture media such as alterations in nutrients, incubation time and temperature, pH, atmospheric conditions etc
- Variable environmental condition rather than constant environment
- Physical factors: radiations
- Chemical factors: Antibiotics, toxins,
- Biological factors: mutagens, bacteriophage.
- Genotypic alterations such as genome rearrangements or mutations.
- Changes in expression of individual genes due to epigenetic phenomenon
Some examples of phenotypic changes
- It involves changes in antigenic structure such as outer lipopolysaccharides (LPS), flagella, pili etc of a bacteria. It is caused due to reversible alterations in specific genomic locus associated with genes for surface structures.
- Modification in antigenic structure assists bacteria in colonizing and establishing the infection by evading host immune system.
- For antigenic modification, the bacteria have evolved a number of molecular mechanisms. Generally, some act by turning individual genes “on” or “off”, while others enable expression of multiple phenotypes via rearrangements of DNA sequences.
- Antigenic variation has been described in a number of bacterial pathogens namely, Salmonella Thyphimurium, Neisseria gonorrhea, Neisseria meningitidis, Haemophilus influenzae and Escherichia coli.
- Example: E. coli have more than 173 O antigen, 56 H antigen and more than 100 K antigen variants.
Competence in Bacillus subtilis
- Development of competence for transformation in B. subtilis is regulated in the nutrients limitation condition and quorum sensing
- Transformation is the process of uptake of naked DNA from media and subsequent recombination of transcribed DNA into the bacterial genome.
- subtilis is a natural competent bacteria, although under optimal condition for transformation, only few bacteria develop competence.
- Competence development in B. subtilis is regulated by expression of Com genes (ComA, ComK, ComE, Com F, ComG). Among which ComK is the key transcription factor regulating transformation which includes DNA-binding, uptake and recombination.
- Expression of ComK is tightly regulated and is induced in response to nutrient limitation and quorum sensing; the ability of bacteria to communicate and coordinate behaviour via signaling molecules.
Endospore in Bacillus subtilis
- Sporulation is the process of formation of a dormant spore during unfavorable conditions which exhibits extreme resistance to environmental conditions.
- subtilis form endospore under nutrient limiting conditions.
- During nutrients deficient condition only some cells activate Spo0A gene. Spo0A gene encodes small acid soluble proteins (SASPs).
- SASPs binds to DNA and protect it from potential damage caused by radiation, desiccation, and drying during unfavorable conditions.
- Sporulation is energy consuming state. SASPs also have killing effects on non-sporulating vegetative cells. Nutrients that are released by the non sporulating cells can be used by the sporulating cells.
- When nutrition is available, spore germinates.
Colicin production in E. coli
- Colicins are plasmid-encoded bacteriocins (antibiotic), produced by coli
- It has bactericidal effects on closely related species ie enterobactericeae family
- Colicin-producing strains are found with high frequency among natural isolates and have been demonstrated to play a role in intraspecies population dynamics.
- Production of colicins is characteristically encoded by a cluster of three genes: colicin producing gene, immunity gene and lysine gene.
- Colicin production or Regulation of colicin gene is induced in E. coli under starvation condition.
Persistence of antibiotic resistant Staphylococcus
- Under antibiotic treatment (penicillin group), few cell of staphylococcus remain viable even after prolong incubation are known as persisters.
- Those persisters are drug resistant and exhibit reduced growth and can resume growth when antibiotics are removed.
- Staphylococcus become drug resistant due to varieties of mechanism. One of them is biofilm formation.
Phenotypes due to Mutation
- Formation of antibiotic resistant mutants
- Antigenic as well as morphological changes in mutant
- Some mutant shows altered metabolic activities ie. increased or decreased ability to utilize some nutrients.
- Lost or gained the ability to synthesize particular enzyme, pigments, toxins etc.
- Change in the colony morphology, shape size and color,
- Resistant of bacteriophage due to change in antigenic structure.