Bacteria
Bacteria are tiny, single-celled organisms found almost everywhere. They lack a nucleus and other organelles, with their genetic material floating in the cytoplasm. Unlike viruses, they have ribosomes for protein synthesis. Bacteria come in various shapes and sizes, with some causing diseases and others being beneficial. They play vital roles in ecosystems, from decomposing organic matter to fixing nitrogen.
Bacteria are called prokaryotic because they lack a defined nucleus and other membrane-bound organelles found in eukaryotic cells. Instead, their genetic material floats freely in the cytoplasm, without being enclosed within a nuclear membrane. This simpler organization distinguishes them from eukaryotic cells, which have a true nucleus and membrane-bound organelles such as mitochondria and the endoplasmic reticulum.
Bacteria are microscopic with a wide range of sizes from 0.2 µm (micron) to 100 µm (micron). Most bacteria are 0.2 µm in diameter and 2-8 µm in length.
Definition: Bacteria are microscopic ubiquitous, single-celled organisms with sizes ranging from 0.2 µm to 100 µm, containing ribosomes for protein synthesis and genetic material, either DNA or RNA, located in the cytoplasm.
Structure of Bacterial Cell
Classification of Bacteria
a. On the basis of Shape
b. Based on composition of cell or Gram Staining test
Gram- Positive Bacteria: Gram-positive bacteria possess a thick layer of peptidoglycan in their cell walls, which retains the crystal violet dye and appears purple or blue-violet under the microscope after staining.
Example of gram positive bacteria are Bacillus, Listeria, Streptococcus, clostridium, Staphylococcus, Corynebacterium, etc
Gram Negative Bacteria: They are a group of bacteria that have a thin layer of peptidoglycan in their cell walls, which is surrounded by an outer membrane. This outer membrane contains lipopolysaccharides (LPS), which are important for protection and defense.
When we stain Gram-negative bacteria using the Gram staining technique, they don’t hold onto the purple color from the crystal violet dye very well. Instead, when we wash them with alcohol during the staining process, the purple color gets washed away. Then, when we add the counterstain (usually safranin), they pick up the pink or red color from it.
Example of gram negative bacteria are Proteus, Salmonella, Pseudomonas, Citrobacter, Enterobacter, etc.
| Basis | Gram Negative Bacteria | Gram Positive Bacteria |
|---|---|---|
| Stains during Gram Staining | Stains red/pink during gram staining test | Stains violet / purple during gram staining |
| Cell Wall Thickness | Thin | Thick |
| Peptidoglycan Layer | Thin | Thick |
| Mesosomes | Absent or rarely present | Present |
| Fimbriae or Pili | Present | Absent |
| Spore Formation | Forms exospores | Forms endospores |
| Toxin Production | Produce endotoxins | Produce exotoxins |
| Outer Layer | Presence of outer layer | Lack of outer layer |
| Lipopolysaccharides (LPS) | Present | Absent |
| Antibiotic Resistance | More resistant | Less resistant |
| Color Retention in Decolorizing Agent | Loses crystal violet stain | Holds crystal violet stain |
c. Based on Mode of Nutrition
- Autotrophic bacteria: Autotrophic bacteria can produce their own organic compounds from inorganic sources. They receive energy through processes such as photosynthesis and chemosynthesis.
Types:
a. Photoautotrophic bacteria: These bacteria use sunlight as their energy source for photosynthesis. Examples include Cyanobacteria.
b. Chemoautotrophic bacteria: They use chemical energy for assimilation, meaning they obtain both their energy and carbon from inorganic sources through chemosynthesis. These bacteria are typically found in environments where sunlight is scarce or absent, such as deep-sea hydrothermal vents, volcanic hot springs, and certain underground habitats.
Examples: Thiobacillus spp., Nitrobacter spp., Methylococcus capsulatus, Nitrosomonas spp. - Heterotrophic bacteria: Heterotrophic bacteria are organisms that obtain energy by consuming organic compounds, but unlike autotrophic bacteria, they do not convert organic compounds into inorganic substances. Instead, they depend on pre-existing organic molecules for their energy needs. Heterotrophic bacteria includes parasitic and symbiotic types, which often have close associations with other organisms.
It can be divided in sub-group as:
a. Saprophytic bacteria: These types of bacteria decompose dead organic matter, recycling nutrients back into the environment. They play a crucial role in nutrient cycling and the breakdown of organic waste. They are decomposers and feed on dead plants and animals. Examples of such types of bacteria include many soil bacteria and certain species of Bacillus.
b. Parasitic Bacteria: Parasitic bacteria obtain nutrients from living organisms, often causing harm or disease. They may live inside the host (endoparasites) or on its surface (ectoparasites). Examples include Mycobacterium tuberculosis, which causes tuberculosis, and Escherichia coli, which can cause various infections.
d. On basis of oxygen requirements or modes of respiration
- Aerobic bacteria: Aerobic bacteria require oxygen for their respiration process and can’t survive in anoxic environments.
Examples: Mycobacterium tuberculosis, Pseudomonas aeruginosa, etc. - Anaerobic bacteria: Anaerobic bacteria do not require oxygen for their respiration process. They respire anaerobically and can’t survive in an oxygen rich environment.
Examples: Clostridium tetani, Bacteroides fragilis, etc. - Facultative anaerobic bacteria: Facultative anaerobic bacteria can grow both in the presence or absence of oxygen. In the presence of oxygen, they perform aerobic respiration, but in the absence of oxygen, they switch to fermentation or anaerobic respiration.
Examples: Escherichia coli, Staphylococcus aureus, etc. - Microaerophilic Bacteria: Microaerophilic bacteria require oxygen, but at lower concentrations than found in the atmosphere. They typically grow best in environments with reduced oxygen levels.
Examples include Campylobacter jejuni and Helicobacter pylori. - Aerotolerant Anaerobes: Aerotolerant anaerobes can tolerate the presence of oxygen but do not use it for respiration. Instead, they rely on fermentation to generate energy. Examples include Lactobacillus species.
e. On the basis of optimum temperature
- Psychrophiles bacteria: Psychrophilic bacteria thrive in cold temperatures and have an optimum growth temperature of 15°C or below. They are commonly found in polar regions, glaciers, and deep ocean waters. Examples include species of Psychrobacter and Moritella.
- Mesophiles bacteria: Mesophilic bacteria prefer moderate temperatures and have an optimum growth temperature between 15°C and 45°C. They are the most common type of bacteria and are found in various environments such as soil, water, and the human body. Many pathogenic bacteria fall into this category. Examples include Escherichia coli, Staphylococcus aureus, and Bacillus subtilis.
- Thermophiles bacteria: Thermophilic bacteria can survive in high temperatures and have an optimum growth temperature above 45°C. Examples include species of Thermus and Geobacillus.
f. On the basis of Habitat
- Halophiles Bacteria: These bacteria thrive in high-salt environments, such as saline lakes, salt flats, and salted foods. They have adapted to survive in conditions where salt concentrations are higher than those found in most environments. Examples include species of Halobacterium and Halococcus.
- Acidophiles Bacteria: Acidophilic bacteria are adapted to thrive in acidic conditions, with pH levels typically below 3. They are found in environments such as acidic soils, acid mine drainage, and acidic hot springs. Acidophiles have capabilities that keep intracellular pH levels stable and protect them from the harmful effects of acidity. Examples include species of Acidithiobacillus and Ferroplasma.
- Alkaliphiles Bacteria: Alkaliphilic bacteria are found in alkaline environments, with pH levels above 9. They are commonly found in habitats such as alkaline lakes, soda lakes, and alkaline soils. Examples include species of Bacillus and Vibrio.
Useful Bacteria
Not all bacteria are harmful to humans. There are some bacteria which are beneficial in different ways:
- Conversion of Milk into Curd: Lactobacillus bacteria, also known as lactic acid bacteria, transform milk into curd through fermentation.
- Fermentation of Food Products: Streptococcus and Bacillus bacteria play very important roles in fermenting various food items like pickles.
- Digestive Health and Immunity: Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria contribute to digestion and improves the body’s immune system.
- Production of Antibiotics: Certain soil bacteria produce antibiotics vital for treating and preventing bacterial infections.
- Nutrient Cycling: Bacteria aid in breaking down organic matter, recycling essential nutrients like carbon, nitrogen, and phosphorus, supporting plant growth.
- Food Production: Bacteria are integral to fermentation processes used in making cheese, yogurt, pickles, and more.
- Medicine Production: Bacteria are used to create a variety of medicines.
- Wastewater Treatment: Bacteria are utilized in wastewater treatment plants to degrade organic matter and pollutants, thereby purifying water.
- Composting: Bacteria play a crucial role in the decomposition of organic waste, helping in the composting processes.
Control of bacteria
Sterilizing or disinfecting exposed surfaces, instruments, and tools is a highly effective means of eliminating or controlling the majority of disease-causing bacteria.
Some methods or techniques of controlling or destroying bacteria includes:
- Application of Heat: Heat is an effective method for killing bacteria by denaturing their proteins and disrupting their cell membranes. Techniques such as autoclaving (steam sterilization), boiling, and incineration can be used to achieve sterilization in medical equipment, laboratory glassware, and food processing
- Disinfectants: Chemical disinfectants such as bleach (sodium hypochlorite), hydrogen peroxide, alcohol, and quaternary ammonium compounds are commonly used to kill bacteria on surfaces and in the environment. These agents work by disrupting bacterial cell membranes, denaturing proteins, or interfering with metabolic processes
- UV Radiations: Ultraviolet (UV) radiation has germicidal properties and can effectively kill bacteria by damaging their DNA. UV lamps are used in water treatment plants, air purification systems, and laboratory settings to disinfect surfaces and equipment.
- Pasteurization: Pasteurization is the process of heating liquids like milk and fruit juice to a specified temperature for a defined amount of time in order to destroy harmful bacteria while preserving flavor and nutritional value. This method helps prevent foodborne illnesses caused by microorganisms such as Salmonella and Escherichia coli (E. coli).
- Boiling: Boiling water is a simple and effective method for killing bacteria and making it safe for drinking and cooking. Boiling water at a rolling boil for at least one minute (or three minutes at higher altitudes) can eliminate most disease-causing bacteria, viruses, and parasites.
Examples of bacteria
Bacteria are ubiquitous microorganisms (i.e. found everywhere, ranging from soil and water to the human body)with both harmful and beneficial roles. These examples given below shows both the harmful and beneficial roles bacteria play in various scopes like human health, agriculture, industry, and the environment.
Disease Causing
- Salmonella – Causes food poisoning.
- Vibrio cholerae – Causes cholera.
- Mycobacterium tuberculosis – Causes tuberculosis (TB).
- Clostridium botulinum – Produces a toxin causing botulism, which leads to paralysis.
- Haemophilus influenzae – Can cause ear infections.
- Streptococcus mutans – Responsible for dental cavities.
- Escherichia coli (E. coli) – Can cause food poisoning and various stomach issues.
- Staphylococcus aureus – Causes skin infections, pneumonia, and food poisoning.
- Helicobacter pylori – Associated with gastritis and peptic ulcers.
- Clostridium difficile – Can cause severe diarrhea and colitis, often associated with antibiotic use.
- Neisseria gonorrhoeae – Causes the sexually transmitted infection gonorrhea.
- Legionella pneumophila – Causes Legionnaires’ disease, a severe form of pneumonia.
- Bacillus anthracis – Causes anthrax, a potentially fatal disease affecting humans and animals.
- Listeria monocytogenes – Can cause listeriosis, a foodborne illness with symptoms ranging from mild flu-like symptoms to severe infections.
Useful Bacteria
- Rhizobium – Forms symbiotic relationships with leguminous plants, aiding in nitrogen fixation.
- Bifidobacteria – Found in the human gut, aiding in digestion and promoting gut health.
- Acetobacter – Used in fermentation processes, such as in the production of vinegar.
- Streptomyces – Known for producing antibiotics and other bioactive compounds.
- Lactobacillus – Used in the fermentation of dairy products like yogurt and cheese, as well as in probiotic supplements.
- Streptococcus thermophilus – Used in the production of yogurt and other fermented dairy products.
- Thiobacillus ferrooxidans – Used in biomining processes to extract metals from ores
- Bacillus thuringiensis (Bt) – Used as a biological pesticide to control insect pests.
- Pseudomonas putida – Known for its ability to degrade various environmental pollutants.
- Cyanobacteria – Photosynthetic bacteria that play a crucial role in oxygen production and nitrogen fixation.
- Propionibacterium freudenreichii – Used in the production of Swiss cheese, contributing to its flavor and texture.
- Bacillus subtilis – Commonly used in the production of industrial enzymes and as a probiotic.
- Acetobacter xylinum – Produces cellulose, used in the production of certain textiles and food products.
- Streptococcus salivarius – Helps maintain oral health by preventing the growth of harmful bacteria in the mouth.
Differences between bacteria and virus
| Basis | Bacteria | Virus |
|---|---|---|
| Cell structure | Bacteria are prokaryotic cells with a simple cell structure lacking a nucleus. | Viruses are acellular entities consisting of genetic material (DNA or RNA) surrounded by a protein coat. |
| Outer cell wall | Bacteria possess a cell wall composed of peptidoglycan, providing structural support | Viruses lack a cell wall. Instead, their genetic material is encapsulated by a protein coat called a capsid. |
| Size | Bacteria are relatively larger in size compared to viruses | Viruses are significantly smaller in size. |
| Living or Non living | Bacteria are living organisms, capable of independent metabolic activities. | Viruses are considered non-living entities since they lack metabolic functions and can only replicate within host cells. |
| Mode of reproduction | Bacteria reproduce through binary fission, a form of asexual reproduction. | Viruses insert their genetic material into host cells and grow and survive within them. |
| Host dependence | Bacteria can reproduce independently of host cells. | Viruses require host cells for replication. |
| Host range | Bacteria have a broad host range and can infect plants, animals, and humans. | Viruses have a narrow host range and typically infect specific species or cell types. |
| Ribosomes | Bacteria contain ribosomes. | Viruses do not possess ribosomes. |
| RNA and DNA | Bacteria contain RNA and DNA in their cytoplasm. | Viruses encapsulate RNA or DNA within a protein capsid. |
| Structure of Genetic Material | Bacterial genetic material is typically a single circular chromosome located in the nucleoid region. | Viral genetic material can be DNA or RNA and may be single-stranded or double-stranded. |
| Infections | Bacterial infections often remain localized, such as pneumonia. | Viral infections, such as the flu, tend to spread systematically throughout the body. |
| Diseases | Diseases caused by bacteria include pneumonia, typhoid, meningitis and food poisoning. | Viral diseases include the common cold, polio, hepatitis, and AIDS. |
| Treatment | Bacterial infections can be treated with antibiotics. | Viral infections are typically managed with vaccines and antiviral drugs. |
| Examples | Examples of bacteria include Salmonella typhi, Vibrio cholerae, and Staphylococcus aureus. | Examples of viruses include coronaviruses, the Tobacco Mosaic Virus (TMV), HIV, and Hepatitis viruses. |
Growth of Bacteria
Bacterial growth or growth of bacteria refers to an increase in the number of bacterial cells and/or the mass of bacterial biomass over time.
Microbial growth curves are graphical representations of the growth patterns of microorganisms over time under specific environmental conditions. They typically consist of four distinct phases: lag phase, exponential (log) phase, stationary phase, and death phase.
The microbes i.e. bacteria is divided by binary fission. Binary fission is the primary method of reproduction for bacteria, as well as for some other single-celled organisms like archaea and certain protists. In binary fission, a single bacterial cell divides into two daughter cells, each receiving a copy of the genetic material and other cellular components. These daughter cells are typically assumed to be identical in all relevant properties, at least in ideal conditions.
- Initial Lag phase: In this initial lag phase, microorganisms are adjusting to their new environment. While they are metabolically active, there is little to no visible growth as they adapt to the conditions.
- Log or exponential phase of growth: Once the microorganisms have adapted to environment or conditions, they enter a phase of rapid growth where they multiply at an exponential rate. This phase is characterized by a steep upward slope on the growth curve as the population expands rapidly.
- Maximum stationary phase: As the available resources (i.e. food for bacteria to survive) in the environment reduces and waste products accumulate, the growth rate slows, and the population reaches a plateau. In this phase, the rate of cell growth equals the rate of cell death, leading to a relatively stable population size.
- Death phase or phase of decline: Eventually, the microorganisms exhaust essential nutrients and accumulate toxic waste products to the point where the rate of cell death exceeds the rate of cell growth. This results in a decline in the population size, forming the downward slope of the curve.