BACTERIA

 

Bacteria

Introduction

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

 General Characteristics

·                 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.

·                  Bacteria are microscopic, unicellular, and they may occur singly or in aggregations to form colonies.

·                  Bacteria possess a rigid cell wall made up of peptidoglycan. The cell wall provides shape, strength, and protection against osmotic pressure.

·                  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.

·                  Well-defined nucleus is absent. i.e. DNA is not enclosed in a nuclear membrane.

·                  Bacteria typically contain a single circular chromosome made of double-stranded DNA.

·                  They may also possess extra-chromosomal DNA called plasmids, which often carry genes responsible for antibiotic resistance or virulence factors.

·                  Bacteria reproduce mainly by binary fission, an asexual process that results in two identical daughter cells.

·                  True sexual reproduction is lacking,  but occurs by conjugation , transformation, and transduction.

·                  Ribosomes are present and are of the  70S type.

·                  Bacteria exhibit various shapes such as cocci (spherical), bacilli (rod-shaped), vibrios (comma-shaped), spirilla (rigid spiral), and spirochetes (flexible spiral).

·                  The plasma membrane is invaginated to form mesosome.

·                  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.

·                  Some bacteria are motile due to the presence of flagella.

·                  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. 

 Based on Temperature Requirement

Type	Temperature Range	Example
Psychrophiles	0–20°C	Rare pathogens
Mesophiles	20–45°C	Most human pathogens
Thermophiles	45–80°C	Environmental bacteria

 

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 are organisms that can grow in the presence as well as the absence of oxygen. Example: all Enterobacteriaceae (E.coli)

4.             Aerotolerant anaerobes 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 at the range of 2 to 10% for growth.

 Based on Oxygen Requirement

Type	Oxygen Requirement	Example
Obligate aerobes	Require oxygen	Mycobacterium
Obligate anaerobes	Oxygen is toxic	Clostridium
Facultative anaerobes	Can grow with or without oxygen	E. coli
Microaerophilic	Require low oxygen	Campylobacter
Aerotolerant anaerobes	Do not use oxygen, but tolerate it	Streptococcus

Bacterial Morphology

Bacterial morphology deals with 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, Bacillus, 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 

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

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

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