Bacteria Cell Structure and Classification_Lecture 1-1.pdf

Bacteria Cell Structure, Classification and Growth Conditions. James Otabil [email protected]  Lecture Objectives At the end of this session, students should be able to: Describe the structure and properties of bacteria. Classify bacteria into various gr...

Bacteria Cell Structure, Classification and Growth Conditions. James Otabil [email protected]  Lecture Objectives At the end of this session, students should be able to: Describe the structure and properties of bacteria. Classify bacteria into various groups based on some properties. Discuss the bacterial growth curve as well as factors that favour and disfavour growth. 2  Bacteria Bacteria are simple, prokaryotic organisms whose cells lack a nucleus or nuclear membrane. The bacteria may appear as rods (bacilli), spheres (cocci), or spirals (spirilla or spirochetes). Bacteria reproduce by binary fission and have unique constituents in their cell walls 3  Bacterial Cell Wall All bacteria have a semi-rigid cell wall except mycoplasmas The cell wall gives shape to the organisms and prevents them from bursting, especially since materials in the cytoplasm exert osmotic pressures The chief component of the prokaryotic cell wall is peptidoglycan, a large polymer composed of N-acetylglucosamine and N- acetylmuramic acid The bacterial cell wall is a unique structure which surrounds the cell membrane 4  Structurally, the wall is necessary for: ✓ Maintaining the cell’s characteristic shape: the rigid cell wall compensates for the flexibility of the phospholipid membrane and keeps the cell from assuming a spherical shape ✓ Countering the effects of osmotic pressure: the strength of the wall is responsible for keeping the cell from bursting when the intracellular osmolarity is much greater than the extra cellular teichoic osmolarity ✓ Providing attachment sites for bacteriophages: teichoic acids attached to the outer surface of the wall are like landing pads for viruses that infect bacteria ✓ Providing a rigid platform for surface appendages: flagella, fimbriae, and pili all emanate from the wall and extend beyond it 5  Bacterial Cell Wall There are two major types of walls: ✓ Gram-positive and Gram-negative 6  Bacterial Cell Wall ✓ GRAM-POSITIVE BACTERIA The cell wall of Gram-positive bacteria consists of many polymer layers of peptidoglycan connected by amino acid bridges The peptidoglycan polymer is composed of an alternating sequence of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAMA) Each peptidoglycan layer is connected, or cross-linked, to the other by a bridge made of amino acids and amino acid derivatives The particular amino acids vary among different species The cross-linked peptidoglycan molecules form a network which covers the cell like a grid 7  Bacterial Cell Wall About 90% of the Gram-positive cell wall is comprised of peptidoglycan Polymers of teichoic acid are commonly associated with the peptidoglycan in Gram-positive bacteria 8 GRAM-NEGATIVE BACTERIA The cell wall of Gram-negative bacteria is much thinner, being comprised of only 20% peptidoglycan. Gram-negative bacteria also have two unique regions which surround the outer plasma membrane: -The periplasmic space and -the lipopolysaccharide layer The periplasmic space, which contains a substance called periplasm, separates the outer plasma membrane from the peptidoglycan layer 9 GRAM-NEGATIVE BACTERIA It is a phospholipid bilayer construction similar to that in the cell membrane and is attached to the peptidoglycan by lipoproteins The lipid portion of the LPS contains a toxic substance, called Lipid A, which is responsible for most of the pathogenic effects associated with harmful Gram-negative bacteria 10 GRAM-NEGATIVE BACTERIA Polysaccharides which extend out from the bilayer also contribute to the toxicity of the LPS The LPS, lipoprotein and the associated polysaccharides together form what is known as the outer membrane. Lipopolysaccharides known as endotoxins are part of this outer membrane 11 Bacterial Cell Wall 12 Gram Positive /Gram Negative 13 Bacterial Plasma Membrane/Function 14  Bacterial Cell Appendages FLAGELLA Flagellin (proteins found in flagellum) PILI (fimbria) Pilin (fibrous proteins in pilus) NB: Pilin are used to exchange genetic materials 15 Types of Flagella Arrangement 16  Bacterial Endospores Resting phase of bacteria Resistant to – heat but can be killed at 121oC - chemicals, radiation, – drying, organic solvents Examples of bacteria with spores: Bacillus, Clostridia 17  Methods of Classification 1. Phenotypic classification 2. Genotypic classification 3. Combination of the phenotypic and genotypic methods (Polyphasic approach). 18  Phenotypic classification Morphological Anatomical Staining Nutrition Environmental factors Biochemical reactions Antigenic structures 19 20 21 Anatomical features –capsules 22 Based on cultural characteristics Utilization of carbohydrates -Oxidative eg., Neisseria - Fermentative eg., E. coli Growth rate - Rapid growers –eg., Vibrio cholerae - Slow growers – M. tuberculosis Pigment production – eg., S. aureus 23  Based on Nutrition Autotrophs: obtain carbon from inorganic sources like carbon dioxide (CO2). Heterotrophs: get their reduced carbon from other organisms. 24  Based on Environmental Factors Temperature Oxygen dependence Ph/Salt concentration Atmospheric pressure 25  Temperature Psychrophiles (15-200C) eg., Pseudomonas fluorescens Mesophiles (20-400C) eg., E. coli, S. aureus. Thermophiles (50-600C) eg. Bacillus stearothermophilus Extreme thermophiles (as high as 2500C). Eg. Thermococcuss spp. 26  Oxygen Dependence (1) Aerobe (grow in Obligate ambient aerobes: strictly temperature, require O2 for their require 21% O2 and growth 0.03% of C O2. (Pseudomonas aeruginosa) Microaerophilic (5 -10% of C O2) eg., C. jejuni, H. pylori, H. influenzae, N. gonorrhoeae 27  Oxygen Dependence (2) Facultative anaerobe (capable Obligate of growing either in anaerobe: presence or absence of O2): E. coli 28  pH/Salt concentration Acidophiles eg., Lactobacillus Halophiles/non- acidophilus) halophiles Eg. Halococcus spp. Alkaliphiles eg., Vibrio sp Majority of the medically important bacteria grow best at a pH ranging from 7.2-7.6 29 Bacterial Growth Conditions What is bacteria growth? and what are the conditions that favour or disfavour growth. Discuss the above after class. 30  What is Bacterial Growth? Orderly increase in cytoplasm and other cellular constituents. Chromosomes replicate – 2 daughter cells. Division is by binary fission. 31  When bacteria are cultivated in a liquid medium, they are usually grown in a batch culture or closed system. No fresh medium is provided during incubation, nutrient conc. declines and waste conc. increases. The growth of bacteria can be plotted as the logarithm of cell number versus the incubation time. 32 Batch growth curve 33  Phases of Growth (1) Lag phase - Log phase – latent exponential phase – Cells divide at a – no apparent cell steady state division – Later limited by – viable cells absence of produce nutrients or necessary increase in harmful enzymes for use metabolites of new medium 34  Phases of Growth (2) Stationary – rate of Decline/death - growth decreases cells die rapidly, Finally very little cell marked difference division between viable Variation in cell morphology- inclusion and total count G +ve may change to G-ve Spores may be produced Toxins are produced 35 Chemostat /Continuous Culture 36 Factors affecting growth Nutrients Right temperature pH aerobic or anaerobic environment 37 Thank you 38