Are Bacteria Intracellular? 9 Facts You should Know


In this article we will discuss the intracellular bacteria complete information. The bacteria are microscopic samm organism, free living. Classically it is divided into two categories, intracellular bacteria and extracellular bacteria

The intracellular bacteria are those bacteria which will enter into the cell and replicate into the host cell

Many of the bacteria will survive outside the cell and some of them survive and release potent toxin inside the cell.

The intracellular bacteria further classified as obligate intracellular bacteria and non obligate intracellular bacteria depanding on its entry and survival in the host cell

What is intracellular bacteria?

The intracellular bacteria are require a host cell to replicate and protect themselves from the host immune response. They feed on the cell matter of host cell

The intracellular bacteria invade inside the cell by preventing from phagocytosis, antibodies and complement defence system of host body. The bacteria will invade into the cell by cell membrane lysis and reside into the cell. Most of the intracellular bacteria are pathogenic nature while some are also non – pathogenic to human.

How are bacteria intracellular?

The bacteria which will breaches the mucosa and skin of host cell, further survive, replicate inside the cell and release potent toxin, cause pathogenicity and infect the host.

The intracellular bacteria are well adapted to escape from the host immune response like phagocytosis and antibodies. Some of the species of intracellular bacteria will infect the non-immune cellls like hepatocytes, epithelial and endothelial cells of host cells while other showed preference to the macrophages cells.

Are all bacteria intracellular?

Classically the bacteria is categorised into extracellular and intracellular bacteria. Further intracellular bacteria are classified into facultative intracellular bacteria and obligate intracellular bacteria

Facultative intracellular bacteria are the bacteria which can survive outside the cell under stress or unfavorable conditions but their preferences is always inside the cell for their replication.The obligate intracellular bacteria are those bacteria which only survive and replicate into the host cell environment. They dird or dormant outside the host cell.

Which type of intracellular bacteria are?

The bacteria which will invade inside the cell by directly through bloodstream by bite of vectors such as mosquito, mites and ticks or from injury place. They protect themselves from the host defence system and invade the cell by makeing passage through cell membrane. Once they inside the cell, used the host cell matter for their survival and replicate inside the cell.

The intracellular bacteria produces extracellular digestive emzyme like hylouronidase attack on interstitial sites of connective tissues, collagenase break down the collagen layer of cell, neuraminidase degrade neuraminic acid that present between the epithelial cells of intestinal mucosa and kinase enzyme that prevent clotting of blood.

Intracellular bacteria examples

The examples of obligate intracellular bacteria are Coxiella burnetti, Chlamydia trachomatis, Anaplasma phagocytophilum, Ehrlichia chaffensis, Orientia tsutsugamushi etc. Some of the examples of facultative intracellular bacteria are Bartonella henselae, Francisella tularensis, Brucella spp., Legionella spp., Rhodococcus equi spp., Nocardia spp., Yersinia spp., etc.

There is difficulty in identifying the obligate intracellular bacteria because they donot grow without inside cell environment and it is very difficult to create inside cell environment when we grow them in laboratory.

Chlamydia life cycle from Shutterstock

Intracellular bacteria growth

The intracellular bacteria grow inside the host cell. They keep their host alive as long as possible for their survival and reproduction. For their growth the bacteria need nutrition which is not available in the free form of cell.

For study of growth pattern of intracellular bacteria of nutrients intake, Legionella pneumophila a facultative intracellular bacteria used as a model. Legionella pneumophila took nutrition by proteasomal degradation of host cell. Due to breakdown of host cell protein, obtained amino acids, which is uptake by bacteria as a food. These amino acid provide the bacteria its primary carbon and energy source.

Intracellular bacteria digestion

The bacteria utilize the actin cytoskeleton of host cell for the movement in the cytosol of host cell and project into adjacent cells by the formation of membrane protrusion. The formed membrane protrusion dissolve into vacuoles from which bacteria escapes, gaining access to the adjacent cells cytosol.

There are three strategies for host cell exist by the intracellular bacteria.

  • Induction of programmed cell death including lytic necroptosis, non-lytic apoptosis and pyroptosis pathway
  • Active host cell destruction, including breaching of host cell membrane such as the vacuolar and/or host cell plasma membrane
  • Induced membrane-dependent exit without host cell destruction e.g. by actin tail- mediated protrusions, budding, extrusions, expulsion, exocytosis or ejection through an ejectosome.

Structure of intracellular bacteria

The intracellular bacteria has five essential structural components: ribosomes, cell membrane, a nucleoids (DNA), cell wall and surface layer. There are some architectural regions: motile organs in the form of flagella and pili; a cell envelope consists of capsule, cell wall, plasma membrane and a cytoplasmic region consisting of cell chromosome (DNA), ribosome and various sort of inclusion.

The structural flagella is made up of protein help in movement of bacteria into the host cell. The sex pilus are present that help in mating during DNA transfer by conjugation.

The capsule including slime layer and glycocalyx present outside the bacteria attach to the host cell surface. Capsule help to protect against phagocytosis (whole cell engulfment); reserve of nutrients or protect them from drying out

The cell wall of bacteria is made up of peptidoglycan and various protein that help to maintain the bacteria shapeand act as a barrier. The plasma membrane is made up of phospholipids and proteins act as semi permeable membrane, allow the transport of solute.

Are Bacteria Intracellular
Intracellular bacteria from Shutterstock

Intracellular v/s extracellular bacteria

The intracellular bacteria are the bacteria which invade the host cell and grow, replicate inside the cell and cause pathogenicity.

  • The extracellular bacteria are free-living but the intracellular bacteria replicate either inside the host cell or in an environment niche.
  • The extracellular bacteria will attach to the host cell surface and intracellular bacteria will invade into the cell and release potent toxin
  • The extracellular bacteria do not penetrate the cell membrane while intracellular bacteria penetrate the cell membrane
  • The extracellular bacteria live outside the environment but intracellular bacteria require the living cell to survive.
  • Examples of extracellular bacteria are Escherchia coli, N. meningitidis, Shiegilla sp, S. aureus , intracellular bacteria examples are Chlamydia spp., Anaplasma spp., Ehrlichia spp., Rickettsia spp., Orientia spp.
  • The number of extracellular bacteria is less as compare to the intracellular bacteria population
  • The extracellular bacteria adhere to host cell surface by the means of fimbria which is usually absent in the intra cellular bacteria.

Tanu Rapria

Hi..I am Tanu Rapria, I have completed my Master's in Biotechnology. I always like to explore new areas in the field of Biotechnology. Apart from this, I like to read, travel and photography.

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