BACTERIA

Bacteria

Introduction

Bacteria are microscopic, unicellular, prokaryotic organisms that lack a true nucleus and membrane-bound organelles.

General Characteristics

1. Bacteria are prokaryotic organisms, which means they lack a true membrane-bound nucleus and membrane-bound organelles such as mitochondria, Golgi apparatus, and endoplasmic reticulum.
2. Bacteria are microscopic, unicellular, and they may occur singly or in aggregations to form colonies.
3. Bacteria possess a rigid cell wall made up of peptidoglycan. The cell wall provides shape, strength, and protection against osmotic pressure.
4. Bacteria have a cytoplasmic membrane composed of a phospholipid bilayer with embedded proteins. This membrane is selectively permeable and regulates the movement of substances in and out of the cell.
5. Well-defined nucleus is absent. i.e., DNA is not enclosed in a nuclear membrane.
6. Bacteria typically contain a single circular chromosome made of double-stranded DNA.
7. They may also possess extra-chromosomal DNA called plasmids, which often carry genes responsible for antibiotic resistance or virulence factors.
8. Bacteria reproduce mainly by binary fission, an asexual process that results in two identical daughter cells.
9. True sexual reproduction is lacking, but occurs by conjugation, transformation, and transduction.
10. Ribosomes are present and are of the 70S type.
11. Bacteria exhibit various shapes such as cocci (spherical), bacilli (rod-shaped), vibrios (comma-shaped), spirilla (rigid spiral), and spirochetes (flexible spiral).
12. The plasma membrane is invaginated to form a mesosome.
13. Bacteria show great diversity in nutritional requirements. Some are autotrophic and can synthesize their own food, while most pathogenic bacteria are heterotrophic and depend on organic substances for nutrition.
14. Some bacteria are motile due to the presence of flagella.
15. Certain bacteria can form endospores under unfavorable environmental conditions.

Classification of bacteria based on temperature requirements.

1. Psychrophiles: Psychrophiles grow well at 0°C and have an optimum growth temperature of 15°C or lower; the maximum is around 20°C. They are readily isolated from Arctic and Antarctic habitats; because 90% of the ocean is 5°C or colder, it constitutes an enormous habitat for psychrophiles. E.g, Pseudomonas, Vibrio, Alcaligenes, Bacillus, Arthrobacter, Moritella, Photobacterium, and Shewanella.

2. Mesophiles: Mesophiles are organism that grows best in moderate temperatures, neither too hot nor too cold, typically between 20 and 45 °C. The optimum growth temperature is 37°C. Almost all human pathogens are mesophiles.

3. Thermophiles: Thermophiles are those organisms that can grow at temperatures between 45°C and 80 °C. They often have optima between 55 and 65°C. These organisms flourish in many habitats, including composts, self-heating hay stacks, hot water lines, and hot springs.

Classification of bacteria based on oxygen Concentration

1. Aerobes:- An organism able to grow in the presence of atmospheric O2 is called an aerobe. Bacteria in which oxygen serves as the terminal electron acceptor for the electron-transport chain in aerobic respiration are called aerobes. Examples- Pseudomonas, Aeromonas, Vibrio

2. Anaerobes:- An organism that can grow in the absence of O2 is an anaerobe. They do not need or use O2. In fact, O2 is a toxic substance that either kills or inhibits their growth. The final electron acceptor is an inorganic compound other than oxygen, like nitrate, sulphate, etc. Examples: Clostridium, Bacteroides

3. Facultative anaerobes:- These are organisms that can grow in the presence as well as the absence of oxygen. Example: E coli, Klebsiella.

4. Aerotolerant anaerobes:- Organisms such as Enterococcus faecalis simply ignore O2 and grow equally well whether it is present or not. Example- Streptococcus

5. Microaerophiles:- There are aerobes, such as Campylobacter, called microaerophiles, that are damaged by the normal atmospheric level of O2 (20%) and require O2 levels in the range of 2 to 10% for growth.

Bacterial Morphology

Bacterial morphology deals with the size, shape, and arrangement of bacterial cells.

Size of Bacteria- The size of bacteria varies depending on the species, but most bacteria are microscopic and can only be seen under a microscope.

Size Range:
Width (diameter): about 0.2 – 2.0 micrometers (µm)
Length: about 1 – 10 micrometers (µm)
(1 micrometer = 1/1000 mm)

Examples- Escherichia coli, approximately 1–2 µm long and 0.5 µm wide. Staphylococcus aureus, about 0.5–1 µm in diameter (spherical shape).Bottom of Form

Shape of Bacteria- There are basically three shapes: Cocci, Bacilli, and Spiral.

a) Coccus or Cocci are spherical bacterial cells, and resemble tiny balls. These bacteria are spherical or oval in shape. Based on arrangement, cocci are further classified as-

Singly: Bacteria that appear as single cell is just called as cocci.

Diplococci: These cells are found in pairs and they are found attached. Eg, Neissseria gonorrhoae, Pneumococcus

Streptococcus: These bacteria form long chains and remain attached. Eg. Streptococcus salivarius

Staphylococcus: These bacteria are arranged irregularly in clusters like grapes. Eg. Staphylococcus aureus

Tetrad: a coccus in a group of four. Eg Micrococcus

Sarcina: coccus in a cubical arrangement of cells. Eg. Sporosarcina






b) Bacillus or Bacilli are rod-shaped bacterial cells, and resemble a pill. These are rod-shaped bacteria. Based on arrangement, bacilli are further classified as-

Singly: Bacteria that exist as a single cell, called bacilli

Diplobacilli: These bacteria have two rod-shaped cells that are attached

Streptobacilli: Cells are arranged as long chains in these bacteria, e.g., Bacillus subtilis


Coccobacilli: Bacteria that have a shape intermediate between cocci (spherical) and bacilli (rod-shaped). They appear as very short rods or oval-shaped cells, so sometimes they may be mistaken for cocci under the microscope. Eg- Haemophilus influenza.


Palisades: They refer to a specific arrangement of rod-shaped bacteria in which the cells lie side by side in parallel rows, resembling a picket fence or a row of matchsticks. Eg- Corynebacterium diphtheriae









c) Spiral bacteria have twisted or helical morphology that resembles little corkscrews. Spiral bacteria are, as the name suggests, spiral-shaped. Spiral-shaped bacteria occur in one of three forms: Vibrio, Spirillum, and Spirochete

a) Vibrio are slightly curved or comma-shaped with less than one complete turn or twist in the cell. Eg- Vibrio cholerae.

b) Spirillum (plural, spirilla). A bacterium with rigid spiral (helical) structure, Eg- Campylobacter jejuni and Helicobacter pylori.

c) Spirochete. helical structure and flexible body (not rigid). Eg Treponema pallidum and Leptospira.





General Structure of Bacterial Cell

A typical bacterial cell consists of external structures and internal structures. The external structures include capsule, flagella, pili (fimbriae), and sometimes sheath. The internal structures include cell wall, cytoplasmic membrane, nucleoid, ribosomes, mesosomes, cytoplasm, plasmids, and in some bacteria, spores. Each component performs a specific function essential for bacterial survival, pathogenicity, and reproduction.

Capsule - The capsule is a viscous, gelatinous layer present outside the cell wall in some bacteria. It is composed mainly of polysaccharides, though in some bacteria it may contain polypeptides or glycoproteins. Approximately 98% of the capsule is water and 2% is polymeric material. The capsule is an important virulence factor. It protects bacteria from desiccation and drying and helps in attachment to host tissues. Most importantly, it prevents phagocytosis by white blood cells, thereby helping the bacteria evade the host immune response. Many pathogenic bacteria such as Streptococcus pneumoniae possess capsules that enhance their disease-causing ability.

Flagella- Flagella are long, hair-like helical structures that arise from the bacterial cell wall and provide motility. They are composed of a protein called flagellin and are antigenic in nature (H antigen). A flagellum consists of three main parts: basal body, hook, and filament. The basal body anchors the flagellum to the cell wall and cytoplasmic membrane, the hook connects the basal body to the filament, and the filament extends outward. Based on arrangement, bacteria may be monotrichous (single flagellum), amphitrichous (one flagellum at each end), lophotrichous (tuft of flagella at one pole), or peritrichous (flagella distributed all over the surface except poles).Pili (Fimbriae)- Pili, also known as fimbriae, are short, thin, hair-like projections made of a protein called pilin. They originate from the cytoplasmic membrane and are commonly found in Gram-negative bacteria. Pili are shorter and more numerous than flagella and are non-motile structures. The primary function of pili is attachment to host tissues and surfaces. Many pathogens of the respiratory and urinary tract use pili to adhere to epithelial cells. A specialized type of pilus known as the sex pilus (F pilus) plays a role in bacterial conjugation, allowing transfer of genetic material between bacteria. This mechanism contributes to the spread of antibiotic resistance genes, which is a major clinical concern.

Cell Wall - The bacterial cell wall is a rigid outer covering composed mainly of peptidoglycan, a complex polymer made of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by peptide bridges. The cell wall provides shape, rigidity, and protection against osmotic lysis. Based on cell wall composition, bacteria are classified into Gram-positive and Gram-negative bacteria.

The wall gives the cell its shape and surrounds the cytoplasmic membrane, protecting it from the environment. It also helps to anchor appendages like the pili and flagella.

Gram-Positive Cell Wall- Gram-positive bacteria possess a thick peptidoglycan layer, which constitutes the major component of the cell wall. They also contain teichoic acids, which are polymers of glycerol or ribitol phosphate. Teichoic acids play a role in cell wall maintenance and act as major surface antigens. Due to the thick peptidoglycan layer, Gram-positive bacteria retain the crystal violet stain and appear purple under a microscope.

Gram-Negative Cell Wall- Gram-negative bacteria have a thin peptidoglycan layer and an additional outer membrane. The outer membrane contains lipopolysaccharide (LPS), proteins, and phospholipids. LPS consists of lipid A, core polysaccharide, and O antigen. Lipid A acts as an endotoxin and is responsible for fever and septic shock in severe infections. Gram-negative bacteria do not retain the crystal violet stain and appear pink after counterstaining with safranin. The presence of an outer membrane makes them more resistant to certain antibiotics.

Cytoplasmic Membrane- The cytoplasmic membrane lies beneath the cell wall and encloses the cytoplasm. It is composed of a phospholipid bilayer with embedded proteins. The membrane is selectively permeable and regulates the movement of substances in and out of the cell. In bacteria, the cytoplasmic membrane is also the site of respiration and energy production because bacteria lack mitochondria. It plays a role in transport, secretion, and biosynthesis of cell wall components.

Nucleoid- The nucleoid is the region of cytoplasm where the bacterial chromosome is located. It is not surrounded by a nuclear membrane. The bacterial chromosome is typically a single circular DNA molecule and lacks histone proteins. The nucleoid contains genetic information necessary for replication, metabolism, and cellular functions.

In addition to chromosomal DNA, bacteria may contain plasmids, which are small circular DNA molecules that carry additional genes, often including antibiotic resistance genes.

Ribosomes- Ribosomes are small granular structures freely distributed in the cytoplasm. Bacterial ribosomes are of 70S type and are composed of RNA and proteins. They are responsible for protein synthesis.

Mesosome- Mesosomes were traditionally described as infoldings of the cytoplasmic membrane seen mainly in Gram-positive bacteria. They were believed to be involved in respiration and cell division. However, modern studies suggest that mesosomes may be artifacts formed during sample preparation for electron microscopy.

Cytoplasm- The cytoplasm is a semi-fluid, colloidal substance enclosed within the cytoplasmic membrane. It contains enzymes, nutrients, metabolites, ribosomes, and genetic material. It is the site of metabolic activities, including synthesis of proteins, lipids, and nucleic acids.

Endospore (Spore)- Endospores are highly resistant, dormant structures formed by certain bacteria during unfavorable environmental conditions. The process of spore formation is called sporulation and usually occurs during the late log phase or early stationary phase of growth. Under favorable conditions, spores germinate to form vegetative cells. Endospores are resistant to heat, radiation, disinfectants, and desiccation. Clinically important spore-forming bacteria include Bacillus and Clostridium species.