Structures in Bacteria: Definition, Diagram, Classification, Microbes, Cells

Edited By Irshad Anwar | Updated on Jul 02, 2025 06:01 PM IST

What Is Bacteria?

Bacteria are single-celled microorganisms that belong to the domain prokaryote since they do not have a true nuclear membrane. Bacteria are present just about everywhere, including in hot hydrothermal vents, and the human gastrointestinal tract. Hence, bacteria are important in the different ecosystems since they are known to contribute to nutrient cycling, decay of organic material and easing the balance of ecosystems. They are essential in human health as commensal organisms that help in digestion and as agents involved in biotechnological uses like antibiotic synthesis and fermentation. But there is another group of bacteria that can be pathogenic, and become carriers of diseases that threaten society.

This Story also Contains

What Is Bacteria?
Classification Of Bacteria
Basic Bacterial Cell Structure
Cytoplasmic Structures
External Structures
Specialised Structures
Intracellular Inclusions
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Structures in Bacteria: Definition, Diagram, Classification, Microbes, Cells

Classification Of Bacteria

Bacterial classification is discussed below

Based On shape

Based on shape: From the above classification, the bacterial cells are classified by their shapes as round or oval in shape – Cocci, long and straight – Bacilli, and the s-shaped bacteria – Spirilla.

Based on Gram staining

Based on Gram staining: Be converted into two groups; the first category is called Gram-positive which is the one that retains the crystal violet dye and the second category is called Gram-negative which does not retain the dye.

Other classifications

According to the bacterial oxygen requirements, the bacteria can be again divided into aerobic which requires oxygen for their survival, anaerobic which does not survive in the presence of oxygen and facultative anaerobes which can survive in both conditions.

Basic Bacterial Cell Structure

The cell structure is discussed below:

Cell Wall

It has a thick layer of peptidoglycan in the cell wall and when stained with gram, the purple dye, crystal violet, is retained hence referred to as gram positive. On the other hand, the cell wall of the gram-negative bacteria has a relatively thinner peptidoglycan layer and is located between the inner and the outer membrane that contains lipopolysaccharide in the outer membrane part which makes them differ when stained with gram stain.

Functions Of The Cell Wall

Their structure has the support of the cell wall and protection from pressure changes in osmotic concentration. It also acts as a protective shield for the bacterial cell against unwanted materials and forms part of the structure and firmness of the bacterial cell.

Diagram of Gram-positive vs. Gram-negative bacteria

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Plasma Membrane

The outer layer of the bacterial cell includes a plasma membrane consisting of a phospholipid bilayer with proteins integrated. It creates a selective membrane that acts as a filter that determines what goes into the cell as well as what goes out to maintain a checkerboard in charge of the cell’s internal environment and obtaining information concerning the surrounding area.

Role In Nutrient Transport And Cell Signalling

It allows the shipment of nutrients into the cell and the cancellation of waste merchandise. Some molecular transport across Bacterial membranes occurs through membrane proteins, and about signalling, there are receptors on the bacterial membrane that give the bacteria the ability to note changes in environments leading to phenomena such as Chemotaxis and Quorum sensing.

Cytoplasmic Structures

The cytoplasmic structures are listed below

Cytoplasm

The only other organelle found in bacterial cytoplasm is the ribosome besides circular chromosomes, enzymes and metabolites fill the ground like a gel of bacteria.

Role In Metabolic Activities

Biomolecules are assigned to catalyse biochemical reactions including glycolysis, the citric acid cycle and synthesising amino acids and nucleotides in bacteria to produce energy and regulate their cellular processes.

Nucleoid

The nucleoid in bacteria is an area found in the cytoplasm of the bacteria that has a single circular chromosome. Hence while human cells contain genetic DNA this DNA is not contained within a membrane-enclosed nucleus but is more structured and condensed by proteins. For these and other purposes, the nucleoid also contains proteins that are involved in the processes of DNA replication transcription and repair.

Differences from the eukaryotic nucleus

While the eukaryotic nucleus is surrounded by a membrane and has multiple linear chromosomes the nucleoid of bacteria is not membrane surrounded. It is negative for histones and introns and its hereditary materials are characteristically shorter and denser. The nucleoid is also involved in bacterial cell division where through the breakdown of the cell the replicated DNA is divided between the daughter cells.

Ribosomes

Structure and function in protein synthesis

Ribosomes are cell organelles manufactured from RNA and protein. The ribosome is of two types – prokaryotic and eukaryotic – The prokaryotic ribosome is smaller in size than the eukaryotic ribosome. These include a big and a small portion of the ribosome which combines during translation. Ribosomes are very important structures that synthesize proteins through the process of translation in which the mRNA strand codes for amino acid sequences that in turn fold into functional proteins.

External Structures

The external structures are listed below

Flagella

Flagella are thin structures which protrude out from the bacterial cell surface and usually resemble a whip. These are made up of a protein known as flagellin and are driven by the proton motive force.

Types Of Flagellar Arrangements

Monotrichous: This showed only one flagellum located at the end part of the cell.
Lophotrichous: Conclusions All flagella at one pole of the cell or multiple flagella at one end of the cell.
Amphitrichous: Flagella present in a single number and localized at both ends of the cell.
Peritrichous: Flagella present all over the surface of the cell

Diagram showing different flagellar arrangements

Pili and Fimbriae

Flagella and cilia are hairlike projections that are present on the outer surface of the bacterial cells. They are made of protein structures known as pilin in their structural makeup.

Role In Adhesion And Conjugation

Fimbriae and pili allow bacteria to attach themselves to a surface host cell, or another bacterium. These adhesions play a very significant role during colonisation, biofilm formation and the infection processes.
Pili are also used in conjugation in which DNA (plasmids) is transferred from one bacterial cell to another. Conjugative pili are the structures or tubes which help in the process wherein the donor cell comes very close to the recipient cell and codes that can render resistance to antibiotics or any other positive quality are transferred.

Capsule And Slime Layer

Both capsules and slime layers are external layers of polysaccharides (and in some cases proteins) that surround the bacterial cell wall.

Differences Between Capsule And Slime Layer

Composition: Capsules are layers which are well compacted and close together, while slime layers are individual layers that are not very much in contact with each other.

Function: Capsules shield the bacteria from phagocytosis and drying while slime layers are useful in holding the bacteria to surfaces and obtaining nutrients.

Importance In Bacterial Virulence And Protection

Capsules contribute to the pathogenicity since they transplant the bacteria from the effect of the immune response of the host and allow bacteria to adhere to tissues and thereby cause infection. The slime layers assist bacteria to survive in unfriendly surroundings and combat the antimicrobial opponents.

Specialised Structures

The specialised structures are listed below

Endospores

Endospores are obtained when bacteria go through sporulation since it is a method of surviving unfavourable conditions. They include the DNA body which is heavily compressed and packaged with the spore coat and cortex surrounding it.

Function In Bacterial Survival Under Harsh Conditions

Endospores allow bacteria to be in extreme conditions such as high temperature regimes or lack of humidity and chemical treatments and withstand them because the genetic material and the rest of the cell structures are protected.

Plasmids

Plasmids are small circular, autoreplicating pieces of DNA that occur in addition to the bacterial chromosome. It can self-multiply within the bacterial cell and is common in conjunction with chromosomal DNA.

Role In Antibiotic Resistance And Genetic Variation

Thus, plasmids are essential in bacterial adaptation and existence at large. These organisms often hold genes that code for traits that enable bacteria to become resistant to antibiotics and their impact. Such genes can be of metabolic activities, toxins or other factors making bacteria competitive and suitable for survival in given conditions carried by the plasmids.

Intracellular Inclusions

The intracellular structures are discussed below:

Inclusion Bodies

Inclusion bodies are diverse structures within bacterial cells that serve various functions

Glycogen granules: Used to store glucose as an emergency energy store during the availability of the nutrients to be used in emergencies when the nutrients are inadequate.
Gas vesicles: Supply floatation to control the position of the bacterium in water to ensure adequate aeration and irradiation.

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Importance In Nutrient Storage

In other aspects, the inclusion bodies serve as nutrient granules where glycogen is stored thus allowing a period of starvation or stress in the bacteria. Gas vesicles help in improving the correct positioning in the environment for efficiency in the uptake of energy and use.

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Frequently Asked Questions (FAQs)

1. What are the main structural differences between Gram-positive and Gram-negative bacteria?

In gram-positive bacteria, there is a thick layer of peptidoglycan and the absence of an outer membrane whereas in gram-negative there is a thin layer of peptidoglycan with the inner and outer membranes that contain lipopolysaccharide coating.

2. How does bacterial flagella differ from eukaryotic flagella?

Flagella are appendages in bacterial cells, or filaments made of flagellin protein, transport in bacterial cells is driven by proton motive force with rotary movement. Eukaryotic flagella are slender threadlike structures that are composed of microtubules, organisational subunits of the cell supported by ATP and move in a whiplike manner for cell moving.

3. What is the function of plasmids in bacteria?

They replicate independently and can contain other genes associated with characteristics that include antibiotic resistance, pathogenicity, or other metabolic aspects. They can self-propagate and be transferred from bacterium to bacterium; play a part in genetic variation and evolution.

4. Why are endospores important for bacterial survival?

Endospores can be described as an exogenous resting structure produced in specific circumstances by certain bacteria. They conserve the bacterial DNA and the other important parts from degradation, thus, letting the bacterium withstand various conditions such as high temperature, dryness, and chemical solutions.

5. How do pili and fimbriae contribute to bacterial pathogenicity?

Both pili and fimbriae are protuberances that allow bacteria to attach to host tissues hence colonising and infecting them. They also regulate biofilm production which helps bacteria to survive in the host environment and resist.

6. How do bacterial cells differ from eukaryotic cells?

Bacterial cells (prokaryotes) differ from eukaryotic cells in several ways: they lack a membrane-bound nucleus, have no organelles like mitochondria or endoplasmic reticulum, are generally smaller, and have a cell wall made of peptidoglycan. Their DNA is usually circular and located in the nucleoid region.

7. What are the main structures found in bacterial cells?

The main structures in bacterial cells include the cell wall, cell membrane, cytoplasm, nucleoid (containing DNA), ribosomes, and sometimes flagella or pili. Some bacteria may also have a capsule or slime layer surrounding the cell wall.

8. What is the function of the bacterial cell membrane?

The bacterial cell membrane, or plasma membrane, acts as a selective barrier controlling the movement of substances in and out of the cell. It's also involved in energy production, cell signaling, and anchoring the DNA during replication.

9. How do bacterial cell walls differ between Gram-positive and Gram-negative bacteria?

Gram-positive bacteria have a thick peptidoglycan layer (20-80 nm) outside their cell membrane, while Gram-negative bacteria have a thin peptidoglycan layer (1-3 nm) sandwiched between an inner and outer membrane. The outer membrane of Gram-negative bacteria contains lipopolysaccharides, which contribute to their pathogenicity.

10. What is the role of the S-layer in some bacteria?

The S-layer (surface layer) is a crystalline array of proteins or glycoproteins found on the cell surface of some bacteria and archaea. It can serve as a protective coat, help in cell adhesion, act as a molecular sieve, or provide anchoring sites for other proteins. The S-layer can also contribute to virulence in some pathogenic bacteria.

11. What is the function of teichoic acids in Gram-positive bacteria?

Teichoic acids are polymers of glycerol or ribitol phosphate found in the cell walls of Gram-positive bacteria. They extend from the peptidoglycan layer to the cell surface. Teichoic acids contribute to cell wall rigidity, help regulate cell division, play a role in ion homeostasis, and can act as attachment sites for some enzymes.

12. What is the function of gas vesicles in certain aquatic bacteria?

Gas vesicles are hollow protein structures found in some aquatic bacteria and archaea. They provide buoyancy, allowing these microorganisms to regulate their position in the water column. This helps them find optimal levels of light and nutrients, which is particularly important for photosynthetic species.

13. How do some bacteria form intracellular membranes, and what is their purpose?

Some bacteria, particularly photosynthetic and methanotrophic species, can form intracellular membranes. These are invaginations or vesicles of the cell membrane that increase the surface area for specific metabolic processes. In photosynthetic bacteria, they house photosynthetic complexes, while in methanotrophs, they contain methane-oxidizing enzymes.

14. What are bacterial outer membrane vesicles (OMVs), and what is their significance?

Outer membrane vesicles are small, spherical structures budded off from the outer membrane of Gram-negative bacteria. They contain various molecules including proteins, lipids, and nucleic acids. OMVs play roles in intercellular communication, nutrient acquisition, defense against antimicrobials, and delivery of virulence factors in pathogenic bacteria.

15. How do some bacteria form magnetosomes, and what is their function?

Magnetosomes are membrane-bound crystals of magnetic minerals (usually magnetite) formed by magnetotactic bacteria. They are produced through a complex process involving iron uptake, crystal nucleation, and growth within specialized vesicles. Magnetosomes allow bacteria to orient along Earth's magnetic field lines, helping them navigate to their preferred microenvironments in aquatic habitats.

16. What is the nucleoid in bacteria, and how does it differ from a eukaryotic nucleus?

The nucleoid is the region in a bacterial cell where the genetic material (DNA) is located. Unlike a eukaryotic nucleus, the nucleoid is not membrane-bound and contains a single circular chromosome along with some proteins.

17. How do bacterial ribosomes differ from eukaryotic ribosomes?

Bacterial ribosomes are smaller (70S) compared to eukaryotic ribosomes (80S). They are also more sensitive to certain antibiotics, which is why some antibiotics can target bacterial protein synthesis without affecting human cells.

18. What is the role of the cytoskeleton in bacterial cells?

Although less complex than in eukaryotes, bacteria do have a cytoskeleton. Proteins like FtsZ (similar to tubulin) and MreB (similar to actin) play roles in cell division, maintaining cell shape, and organizing cellular contents. The bacterial cytoskeleton is crucial for growth, division, and spatial organization of cellular components.

19. How do bacteria produce and secrete enzymes?

Bacteria produce enzymes through protein synthesis on ribosomes. Many enzymes are secreted through various secretion systems, which are protein complexes that span the cell envelope. Some enzymes remain in the cytoplasm, while others are transported to the periplasm or fully secreted outside the cell, depending on their function.

20. What are bacterial nanotubes, and what is their function?

Bacterial nanotubes are thin, tubular structures that can form between bacterial cells. They allow for the exchange of cytoplasmic contents, including proteins and even small bits of genetic material. Nanotubes may play roles in communication, nutrient sharing, and possibly the spread of antibiotic resistance genes between cells.

21. How are bacteria classified based on their cell wall structure?

Bacteria are classified into two main groups based on their cell wall structure: Gram-positive and Gram-negative. Gram-positive bacteria have a thick peptidoglycan layer, while Gram-negative bacteria have a thin peptidoglycan layer surrounded by an outer membrane.

22. What is the Gram staining technique, and why is it important?

The Gram staining technique is a method used to differentiate between Gram-positive and Gram-negative bacteria based on their cell wall composition. It's important because it helps in identifying bacteria, choosing appropriate antibiotics, and understanding bacterial physiology and pathogenicity.

23. What is the function of the bacterial cell wall?

The bacterial cell wall provides structural support, maintains cell shape, and protects against osmotic lysis (bursting due to water influx). It also acts as a barrier against certain substances and helps in cell-to-cell communication.

24. What is the function of the periplasmic space in Gram-negative bacteria?

The periplasmic space is the area between the inner and outer membranes in Gram-negative bacteria. It contains enzymes for nutrient breakdown, proteins involved in cell wall synthesis, and components of transport systems. It also acts as a buffer zone between the cell interior and the external environment.

25. How do bacterial toxins relate to cellular structures?

Bacterial toxins can be associated with various cellular structures. Some are released into the environment (exotoxins), while others are part of the cell wall (endotoxins in Gram-negative bacteria). Certain toxins are injected directly into host cells through specialized secretion systems, which are complex protein structures spanning the bacterial cell envelope.

26. What are plasmids, and why are they important in bacteria?

Plasmids are small, circular DNA molecules separate from the bacterial chromosome. They are important because they often carry genes for antibiotic resistance, virulence factors, or other beneficial traits. Plasmids can also be transferred between bacteria, allowing for rapid adaptation.

27. What are inclusion bodies in bacteria?

Inclusion bodies are aggregates of substances within the bacterial cytoplasm. They can be storage granules for nutrients (like glycogen or polyphosphate), gas vesicles for buoyancy, or even crystalline protein aggregates. These structures help bacteria store resources or adapt to their environment.

28. What are magnetosomes, and which bacteria possess them?

Magnetosomes are membrane-bound crystals of magnetic iron minerals found in magnetotactic bacteria. These structures allow the bacteria to orient themselves along Earth's magnetic field lines, helping them navigate to their preferred microenvironments in aquatic habitats.

29. How do bacteria regulate gene expression without a nucleus?

Bacteria regulate gene expression through various mechanisms including operons (groups of genes controlled together), sigma factors (proteins that help RNA polymerase recognize promoter regions), and small regulatory RNAs. They can respond quickly to environmental changes due to the proximity of their DNA to the cellular machinery.

30. How do some bacteria produce endospores, and what is their significance?

Endospores are produced by certain bacteria (like Bacillus and Clostridium species) in response to nutrient depletion. The process involves the formation of a thick, protective coat around a copy of the bacterial DNA and some cytoplasm. Endospores can survive extreme conditions and allow bacteria to persist in harsh environments for long periods.

31. What are flagella, and how do they function in bacteria?

Flagella are long, thin, whip-like structures that some bacteria use for locomotion. They rotate like propellers, allowing bacteria to move towards nutrients or away from harmful substances in a process called chemotaxis.

32. What are pili, and what roles do they play in bacterial cells?

Pili (singular: pilus) are short, hair-like appendages on the surface of some bacteria. They can be involved in bacterial conjugation (transfer of genetic material between cells), adherence to surfaces or host cells, and in some cases, a form of movement called twitching motility.

33. How do bacterial spores differ from regular bacterial cells?

Bacterial spores are dormant, highly resistant structures formed by some bacteria in response to harsh environmental conditions. Unlike regular cells, spores have little to no metabolic activity, contain dehydrated cytoplasm, and have multiple protective layers that make them resistant to heat, radiation, and chemicals.

34. What is the capsule in bacteria, and why is it important?

The capsule is a layer of polysaccharides or proteins outside the cell wall in some bacteria. It's important for protection against phagocytosis by host immune cells, adherence to surfaces, and in some cases, virulence. The capsule also helps prevent the bacteria from drying out.

35. How do bacteria divide, and what structures are involved in this process?

Bacteria divide through binary fission. The process involves DNA replication, chromosome segregation, and the formation of a septum (division plane) at the cell center. The protein FtsZ forms a ring-like structure (Z-ring) at the division site, which constricts to separate the two daughter cells.

36. What are bacterial biofilms, and how do they relate to cellular structures?

Biofilms are communities of bacteria adhered to surfaces and encased in a self-produced extracellular matrix. The formation of biofilms involves changes in gene expression and the production of extracellular structures like pili and adhesins. Bacteria in biofilms often show altered cellular structures and behaviors compared to their planktonic (free-floating) counterparts.

37. How do some bacteria produce light, and what structures are involved?

Some bacteria produce light through a process called bioluminescence. This involves a chemical reaction catalyzed by the enzyme luciferase, which oxidizes a compound called luciferin. The genes for this process are often organized in an operon called the lux operon. The light-producing apparatus is typically located in the cell membrane or in specialized organelles.

38. How do bacterial efflux pumps work, and why are they important?

Efflux pumps are protein structures spanning the bacterial cell envelope that can expel various substances from the cell. They are important for removing toxic compounds, including antibiotics, which contributes to antibiotic resistance. Efflux pumps can be specific for certain molecules or have a broad range of substrates.

39. What are bacterial microcompartments, and what is their significance?

Bacterial microcompartments are protein-based organelles found in some bacteria. They encapsulate specific enzymatic pathways, allowing for the concentration of enzymes and substrates, the sequestration of toxic intermediates, or the creation of a specialized chemical environment. Examples include carboxysomes for carbon fixation and Pdu microcompartments for 1,2-propanediol utilization.

40. How do bacteria produce and utilize storage compounds?

Bacteria can produce various storage compounds, such as glycogen (for carbon storage), polyphosphate (for phosphorus storage), and polyhydroxyalkanoates (for carbon and energy storage). These are typically accumulated as granules in the cytoplasm and can be broken down when nutrients are scarce, helping bacteria survive periods of nutrient limitation.

41. How do bacterial chemoreceptors function, and where are they located?

Chemoreceptors in bacteria are typically transmembrane proteins located in the cell membrane. They detect specific chemicals in the environment and relay this information to the flagellar motor, allowing the bacteria to move towards attractants or away from repellents. This process, known as chemotaxis, is crucial for bacterial survival and adaptation.

42. What are bacterial nanowires, and how do they function?

Bacterial nanowires are electrically conductive appendages produced by some bacteria. They can be pili-like structures or extensions of the outer membrane. Nanowires allow bacteria to transfer electrons over long distances, which is important in processes like extracellular electron transfer in soil and sediment environments.

43. How do bacteria produce and secrete siderophores, and why are they important?

Siderophores are small, high-affinity iron-chelating compounds produced by many bacteria. They are synthesized in the cytoplasm and secreted through specific transport systems. Siderophores are crucial for iron acquisition, especially in iron-limited environments. They bind to iron in the environment and are then taken up by the bacteria through specific receptors.

44. What are bacterial appendages, and how do they differ from each other?

Bacterial appendages are structures that extend from the cell surface. They include flagella (for motility), pili (for adherence and genetic transfer), and fimbriae (for adherence). These structures differ in their composition, size, and function. For example, flagella are long and used for swimming, while pili are generally shorter and involved in attachment or DNA transfer.

45. How do bacteria produce and utilize extracellular polymeric substances (EPS)?

Extracellular polymeric substances are high-molecular-weight compounds secreted by many bacteria. They are primarily composed of polysaccharides, proteins, nucleic acids, and lipids. EPS production involves various biosynthetic pathways and secretion systems. EPS plays crucial roles in biofilm formation, protection against environmental stresses, and cell-to-cell communication.

46. What are bacterial carboxysomes, and how do they function?

Carboxysomes are protein-based microcompartments found in many autotrophic bacteria. They encapsulate the enzymes involved in carbon fixation, particularly RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase). By concentrating CO2 around RuBisCO, carboxysomes enhance the efficiency of carbon fixation, which is crucial for the growth of these bacteria.

47. How do bacteria regulate osmotic pressure, and what structures are involved?

Bacteria regulate osmotic pressure through various mechanisms, including the accumulation or release of compatible solutes (osmolytes) and the use of mechanosensitive channels. The cell membrane and cell wall play crucial roles in maintaining cell integrity under osmotic stress. Some bacteria also produce capsules or S-layers that can provide additional protection against osmotic changes.

48. What are bacterial type III secretion systems, and how do they function?

Type III secretion systems are complex protein structures found in some Gram-negative bacteria, particularly pathogens. They act like molecular syringes, allowing bacteria to inject proteins directly into host cells. These systems span the bacterial cell envelope and can penetrate host cell membranes, facilitating the delivery of virulence factors that can manipulate host cell functions.

49. How do bacteria produce and utilize storage lipids?

Some bacteria can produce and store lipids, typically in the form of triacylglycerols or wax esters. These are accumulated as lipid droplets in the cytoplasm. The synthesis involves specific enzymes like diacylglycerol acyltransferases. Storage lipids serve as energy and carbon reserves, helping bacteria survive periods of nutrient limitation.

50. What are bacterial pili, and how do they differ from flagella?

Pili are thin, hair-like appendages on the surface of many bacteria. Unlike flagella, which are used for motility, pili are primarily involved in adherence to surfaces or other cells, and in some cases, DNA transfer during bacterial conjugation. Pili are generally shorter and more numerous than flagella, and they have a different protein