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Swollen eye with conjunctivitis

Image : “Eye with conjunctivitis exuding pus” by Tanalai. License: CC BY-SA 3.0

Definition of Bacterial Infection

The intrusion of bacteria into a host, either passively or actively, is referred to as an infection (Lat. inficere ‘to put in’) if the proliferation and reaction of the host lead to disease. Bacteria that cause disease in the human body are referred to as human pathogens.

Image: Overview of the main bacterial infections and the most notable species involved. By Mikael Häggström, License: Public Domain

Morphology and Structure of Bacteria

Bacteria range from 0.2–2 µm in size and there are 3 basic bacterial forms comprising the morphology of all eubacteria.

  • Cocci: spherical or oval. Cocci often appear in groups of 2, 4, or 8 cells (diplococci, tetrads, sarcina), in grape-like clusters (staphylococci), or chains (streptococci).
  • Bacillus: rod-shaped, in different manifestations, (slim (e.g., Mycobacterium tuberculosis), plump (e.g., Escherichia coli), and pointed or rounded ends).
  • Helical: have distinctive morphologic distinctive windings; including Spirilla, Borrelia, Treponema, and Leptospira.
  • Pleomorphic: no distinct shape (Chlamydia or Rickettsiae)

Regarding structure, prokaryotes markedly differ from eukaryotic cells. Although the different types of bacteria vary in metabolism, structure, and virulence, bacteria also possess some common structures. The most important organelles and their functions are listed in the table below (The cell membrane will be discussed in the following paragraph because the cell membrane plays an important role in the classification and treatment of bacteria).

Structure Function
  • Double-stranded circular or linear genetic material
  • Approx. 106 base pairs 1000 genes; (human: 6 x 109 base pairs)
  • Localized in the cytoplasm with a connection to the cell membrane (a nucleus does not exist)
  • Circular double-stranded DNA molecules
  • 800–300,000 base pairs
  • Not essential genes of the bacterium but genes have a selective advantage
  • For example, resistances, toxins, virulence features, and realization of specific substances in a habitat
  • Does not contain endoplasmic reticulum or mitochondria
  • Essential enzymes of metabolism are localized in the cell membrane
Cytoplasmic membrane
  • Lipid bilayer
  • Can partially invaginate into mesosomes (membrane bodies), which are filled with enzymes
  • Differs in lipid-protein composition compared to eukaryotes
  • Hypometabolic survival form of bacteria
  • In this state, sporogenic bacteria are able to survive extreme temperatures and other environmental conditions that are normally lethal.
  • Bacilli and clostridia are 2 types of sporogenic bacteria
  • Means of transportation of the cell is subdivided into 3 parts (basal body, hook, and filament).
  • The helical filament is boosted by a sodium/proton gradient. The reaction of the surrounding environment enables taxis.
  • Polar (single; unilateral), lophotrichous (clump; unilateral), amphitrichous (bilateral), or peritrichous (all around) flagellation is possible.
  • Tubular structures through membranes and cell membrane
  • Sexual pili for DNA exchange (conjugation) between 2 bacterial cells
  • Adhesins for attachment of structures in the host organism

Cell Membrane

The most important task of the solid cell membrane is to withstand the high osmotic internal pressure of the bacterium and prevent the cell from bursting. The cell membrane of all bacteria consists of peptidoglycans or murein. This grid consists of the polysaccharides, N-acetylmuramic acid and N-acetylglucosamine, both of which are interconnected through short peptide side chains. To better understand the difference between the 2 subspecies (gram-positive and -negative) the gram stain will be explained first.

The gram stain

Foto einer Gramfärbung

Image: Microscopic image of a Gram stain of mixed gram-positive cocci (Staphylococcus aureus ATCC 25923, purple) and gram-negative bacilli (Escherichia coli ATCC 11775, red). Magnification:1,000. By Y tambe, License: CC BY-SA 3.0

During the gram-staining process, bacteria are colored using crystal violet and Lugol’s solution. Then, the cells are washed intensively with alcohol and counterstained with eosin (red stain).

A thick murein layer prevents the washout of the stains, thus the staining is positive and the bacteria appear blue under the microscope. In the presence of a thin murein layer, the staining is washed out of the cell, thus the bacteria are gram-negative means that they are gramnegative and, due to the red counterstaining, the bacteria appear red under the microscope.

Gram-positive bacteria possess a thick cell membrane that consists of multiple layers of peptidoglycans and a thick murein layer. Various proteins, cell membrane-teichoic acid, and a specific polysaccharide can be anchored in the cell membrane.

Schema zeigt den Vergleich zwischen grampositiven und gramnegativen Bakterien

Image: Schematic of the envelopes of gram-positive (on the right) and gram-negative bacteria (on the left). By Filomena Nazzaro et al., License: CC BY 3.0

Gram-negative bacteria possess a single murein layer on the outside of which another 2nd outer membrane is located. This membrane contains, for example, lipopolysaccharides that have an endotoxic effect when the cell breaks down.

Transpeptidases that connect individual components with each other are crucial to the development of a cell membrane. Transpeptidases are an important target for penicillin and other antibiotics.


This article only discusses human pathogenic bacteria. Human pathogenic bacteria are always chemosynthetic (convert carbon into organic matter) and organotrophic (obtain energy from organic substances). The primary means of producing energy is the oxidation of organic substances and the transfer of electrons, such as H+-ions. This procedure is the same in prokaryotes and eukaryotes. Therefore, bacteria use 2 different metabolic pathways.


Respiration is characterized by the transmission of H+-ions to oxygen. Respiration can be aerobic or anaerobic, where oxygen is chemically bound in a type of salt. The energy yield of respiration is approx. 10 times greater than the yield of fermentation.

Fermentation (zymosis)

In the absence of oxygen, another organic compound serves as a hydrogen acceptor. The end-products of this process are alcohol, carbon dioxide, and lactic acid.

The above results in the following classification of bacteria based on metabolism:

  • Facultative anaerobes: bacteria can perform aerobic and anaerobic respiration.
  • Obligate aerobes: bacteria are reliant upon aerobic respiration and thus require oxygen in the environment.
  • Obligate anaerobes: bacteria are well- adjusted to anaerobic respiration. Metabolism is inhibited when brought into contact with oxygen. This results in cellular death.
  • Aerotolerant anaerobes: bacteria are well- adjusted to anaerobes respiration and tolerate low levels of oxygen because they have superoxide dismutase. 

Proliferation of Bacteria—Reproduction and Gene Transfer

Bacteria multiply by simple cell division. The 2 daughter cells that result from the original cell have 2 identical (apart from incidental mutations) copies of the genomes. This vertical gene transfer from 1 generation to the next appears clonal.

Therefore, the recombination of genomes as in meiotic reproduction is not possible. Bacteria achieve genetic diversity through horizontal gene transfer, which means the exchange of information between 2 individuals of the same generation. Exchange without meiosis is also referred to as parasexuality.


Transformation is the absorption of naked DNA fragments from lysed bacteria. This occurs without the need for vectors, bridges, or other chemical tools. Some bacterial species possess this natural competence of DNA absorption. Using receptors, bacteria acknowledge DNA strands on the cell surface, and fragment DNA stands into short, single-stranded pieces, and phagocytose DNA stands into the cell interior. If there is a homologous gene segment, the external DNA can mate and take the new information into the genome. This type of gene transfer takes place only within 1 species because the bacteria recognize species-specific DNA.


Transduction occurs when a virus enters a cell and has bacterial genetic material (bacteriophage) in the capsid. This new piece of genetic material takes over the cell and the bacteria will begin to produce the bacteriophage, viral capsid, and enzymes. Eventually, the cell will be filled with viral components and bacteriophages and will lyse. The virus then proceeds to infect another bacterium.


Via so-called conjugation bridges, individual plasmids between 2 cells are exchanged. The F-plasmid contains the genes that allow the formation of a conjugation bridge and the passing of a plasmid. The bacterium that contains the F-plasmid represents the donor cell and its partner, the recipient. The donor cell forms F-Pili (pilus – Latin ‘hair’), which establishes contact with the recipients.

Afterward, the pili shorten and fuse, so that a pore or a short bridge is present between 2 bacterial cells. Due to the rolling-cycle mechanism, a single strand of the plasmid can be synthesized, which is then later completed in the recipient to a double F-plasmid form. The recipient now can also pass as a donor of the F-plasmid.

Das Schema zeigt bakterielle Konjugation

Image: Schematic drawing of bacterial conjugation. By Adenosine, License: CC BY-SA 3.0

This gene transfer is only unidirectional (in 1 direction). Donors and recipients are sometimes abbreviated with F+ and F in the literature.

Growth curve of a bacterial population

The proliferation of a bacterial population follows a typical growth curve. It can be observed by exposing a particular number of bacteria in nutrient agar. After an initial slow cellular growth and adaptation to a new environment (lag phase), the population starts to rapidly divide, which quickly results in exponential growth through numerous cell divisions (log phase). During the deceleration phase, the cell growth slows down and culminates in a stationary phase. The bacteria count increases to up to 109 cells/mL. Due to nutrient exhaustion and increased toxic degradation from cellular overcrowding, the death phase commences.

Bakterielle Wachstumskurve

Image: Bacterial growth. By M•Komorniczak, License: CC BY-SA 3.0

The exponential growth of bacteria shows that the treatment of bacterial infections should be initiated quickly and that waiting can cause a proliferation of pathogens with fatal consequences. However, the growth rate depends on the supply of nutrients and other environmental conditions.

To estimate the growth rate of bacterial strains, generation time (T), which represents the time it takes a parent cell to divide into 2 daughter cells or the time for a doubling of the bacterial population should be determined.

Generation time can be determined during the exponential growth phase (log phase). Therefore, the initial number of bacteria n0, and the number of bacteria (n) at a specific time (t) can be determined in the experiment. Because growth is exponential, the equation is:

n = nx 2T/t

T/t indicates the number of generation times or divisions.

Using logarithms, the equation is:

log n  = log n0 + t/T × log 2   = log n0 + 0.301 × t/T   = log n0 + 0.301/T  × t

Applying the logarithm, logarithm, the current equation corresponds to a linear equation. The graph of the log cell number against time,t, corresponds to a straight line with a slope 0.301/T. The generation time can now be read from the graph.

Pathogenicity and Virulence Factors

The pathogenicity of a bacterial strain is referred to as the ability of a bacterial species to cause disease. To classify the distinctiveness of this ability, virulence is used. Virulence factors include:

  • Cell wall components, which are recognized as antigens
  • Surface proteins or capsules
  • Excreted exotoxins
  • Excreted metabolites
  • Metabolites/endotoxins released after cell death

The virulence of different species can differ widely. Virulence is specified using LD50 (lethal dose 50), which represents the dose at which 50% of an infected test group dies. In bacteria with strong virulence, there are only minor differences between LD50 and the dose at which 100% of the individuals in a test group die. An example of this phenomenon is Streptococcus pneumoniae. The LD50 cannot be determined because only a few cells of S. pneumoniae are able to kill an entire test group. In contrast, there is a clear demarcation between the LD50 and LD100 of Salmonella enterica. Pathogen dose of approx. 100-fold more 100 times more is required to kill the entire test population than to kill 50% of the test objects.

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