Bacteria: Structure, Reproduction, Illnesses

Brief overview

  • Bacteria – Definition: microscopic unicellular organisms without a cell nucleus
  • Are bacteria living organisms? Yes, because they fulfill necessary criteria (such as metabolism, growth, reproduction).
  • bacterial reproduction: asexual by cell division
  • bacterial diseases: e.g. tetanus, diphtheria, whooping cough, scarlet fever, chlamydial infection, gonorrhea, bacterial tonsillitis, bacterial pneumonia, bacterial otitis media, salmonellosis, listeriosis, tuberculosis, cholera, typhoid, plague
  • Treatment of bacterial infections: Antibiotics
  • Vaccination against bacteria: possible e.g. for diphtheria, whooping cough, tetanus, meningococcal and pneumococcal infections, cholera, typhoid fever

What are bacteria?

Bacteria are microscopic, single-celled organisms and the oldest living organisms on earth. They occur in numerous different species and are found virtually everywhere in the world – in the air, water and soil, deep inside the earth’s crust and on the tops of the highest mountains, in hot springs, and in the Arctic and Antarctic.

Bacteria make up by far the largest proportion of the human normal flora (plus a few others such as fungi and parasites). Normal flora refers to all microorganisms that naturally colonize the body. If experts consider only one specific site of colonization, they speak, for example, of intestinal flora (totality of all natural bacteria in the intestine).

In addition, there are a few species of bacteria that can cause diseases in humans. These types of human pathogenic bacteria make up only about one percent of all known bacterial species.

Structure of bacteria

Bacteria range in size from 0.1 to 700 micrometers (one micrometer = one thousandth of a millimeter). This makes bacteria much larger than viruses, but in most cases still smaller than human cells.

Cell wall and flagella

In many cases, the bacterial cell wall is rigid, thus giving the bacterium a fixed shape (e.g., spherical and rod-shaped bacteria). In addition, there are helical bacteria with a thinner and relatively flexible cell wall. This allows the bacterial cell to move around with helical (and other) movements. Bacteria with a rigid cell wall, on the other hand, usually have long, filamentous flagella with which they can move (see below: Classification by flagella).

There are also a few bacteria without cell walls. Examples are the mycoplasmas (parasitic bacteria that can nevertheless reproduce themselves) and thermoplasma species (heat-loving bacteria with a stable plasma membrane that live in volcanic soils, for example).

Capsule

Most bacteria additionally surround themselves on the outside with a capsule (see below: classification according to encapsulation). This is a relatively sharply defined, very dense protective layer of sugars or protein building blocks (amino acids).

Cell membrane and cytoplasm

Inside the cell wall of a bacterial cell, a cell membrane is attached, as it is found with a similar structure in animal (including human) cells. Some bacteria also have an outer cell membrane. It surrounds the cell wall.

Inside the cell, i.e. in the cytoplasm, the genetic material of the bacterial cell, the so-called bacterial genome, is found along with various other cell structures (such as the so-called ribosomes for protein synthesis). Sometimes bacteria contain additional genetic material in the form of plasmids.

Bacterial genome

The bacterial genome contains all the genetic information of the bacterial cell necessary for life (information on structure, metabolism, reproduction). It consists of double-stranded DNA (abbreviation for deoxyribonucleic acid), i.e. a double-stranded chain of certain sugar and other building blocks. The genetic material of animal cells is also made up of DNA. However, there are significant differences between animal and bacterial cells:

  • Animal cells: The DNA genome is located separately from the rest of the cytoplasm in its own membrane-enclosed compartment – the nucleus. In addition, it is linearly organized, i.e. it is present in the form of individual chromosomes (quasi individual DNA threads).

Plasmids

In addition to the bacterial chromosome, the cytoplasm of some bacteria contains other small, double-stranded DNA rings, either single or multiple, known as plasmids. They contain genetic information that a bacterial cell does not need under normal living conditions, but which may give it a survival advantage under difficult conditions.

This can be, for example, the blueprint for a toxin that kills other bacteria. The ability of a bacterial cell to be resistant to a particular antibiotic may also be stored in plasmids.

Antibiotics are drugs that are specifically effective against bacteria. They are therefore part of the standard therapy of a bacterial infection.

Plasmids are replicated independently of the bacterial chromosome and distributed more or less randomly to the two daughter cells when a bacterium multiplies by cell division.

Conjugation takes a few minutes, but is only possible between certain types of bacteria.

Bacteria vs. viruses

The most important difference is that bacteria have a metabolism and can reproduce independently – this is not true for viruses. Read more about the comparison between viruses and bacteria in the article Viruses.

Which bacteria are there?

Currently, about 5,000 species of bacteria are known. In fact, however, there are probably many more: experts suspect that there are hundreds of thousands of different types of bacteria in the world.

The germs can be classified according to various criteria; the most common are:

Classification by coloration

Bacteria can be classified according to the color they take on when they come into contact with certain staining agents. The most common staining method for identifying bacteria is called Gram staining. According to this, a distinction is made between:

  • Gram-positive bacteria: They turn blue after the addition of a certain chemical substance. Examples include the diphtheria and anthrax pathogens, pneumococci (cause pneumonia, meningitis, sinusitis, and otitis media, for example), and streptococci (possible triggers of pneumonia and tonsillitis, among others).
  • Gram-negative bacteria: They take on a red color when Gram-stained. Examples are the pathogens of whooping cough, typhoid, cholera and plague.

The different wall structure also has practical consequences for medicine, namely when it comes to the treatment of bacterial infections: some antibiotics are only effective against Gram-positive bacteria, others only against Gram-negative bacteria.

Classification according to form

There are three basic bacterial forms:

  • spherical bacteria: These roundish to oval bacteria (also called cocci) often cluster together in typical ways: in groups of two, four, or eight, in larger clusters (staphylococci), or as more or less long chains (streptococci).
  • Rod-shaped bacteria: The slender or plump rod-shaped bacteria may be present singly (such as typhoid bacteria) or in different bearings to each other (such as diphtheria bacteria). Rod-shaped bacteria that need oxygen to live (aerobic) and can form spores (see below) are also called bacilli (such as anthrax bacteria).
  • helical bacteria: According to their exact appearance, these bacteria are divided into four groups – spirilla (e.g., the causative agent of rat bite fever), borrelia (e.g., the causative agent of Lyme disease), treponema (e.g., syphilis bacteria), and leptospira (e.g., the causative agent of leptospirosis).

Classification according to pathogenicity

  • facultative pathogenic bacteria: These bacteria cause disease only under certain circumstances, such as when the immune system is weakened.
  • obligate pathogenic germs: In sufficient quantity, they always cause a disease, for example salmonella.

Bacteria that occur naturally in the body can also cause illness – for example, if they spread excessively as a result of a weak immune system or get into the wrong places in the body (e.g. intestinal bacteria that get into the urethra or vagina as a result of incorrect toilet hygiene). They thus belong to the facultative pathogenic bacteria.

Classification according to flagella

Most bacteria carry flagella on their outer surface, with the help of which they are mobile. Experts distinguish between the following forms of flagellation:

  • monotrichous flagellation: only one flagellum, e.g. cholera bacteria
  • lophotrichous flagella: several flagella arranged in one or two tufts, e.g. Pseudomonas species
  • peritrichous flagella: several flagella distributed over the entire outer surface of the bacterial cell (flagella all around), e.g. Salmonella (causative agent of salmonellosis and typhoid fever)

Classification according to encapsulation

The bacterium Haemophilus influenzae, for example, is encapsulated. Among other things, it can cause meningitis, otitis media, bronchitis, pneumonia and – as Haemophilus influenzae type B (HiB) – laryngitis.

Also among the encapsulated forms of bacteria are pneumococci (Streptococcus pneumoniae). They typically cause pneumonia, but sometimes other bacterial infectious diseases.

Classification according to spore formation

Under unfavorable living conditions, some bacteria can form permanent forms with massively reduced metabolism – so-called spores. Unlike the metabolically active (vegetative) cell, these can withstand extremely unfavorable environmental conditions such as heat and cold and remain viable for years or even decades. As soon as conditions improve again, the spore transforms back into a vegetative bacterial cell.

Spores are effectively bacteria in a dormant state.

Spore-forming bacteria mainly include representatives of the genera Bacillus and Clostridium, for example the anthrax pathogen (Bacillus anthracis) and the pathogens of tetanus (Clostridium tetani) and botulism (Clostridium botulinum).

Classification according to ratio to oxygen

Obligate anaerobic bacteria (anaerobes) are the exact opposite of obligate aerobes: They cannot grow and thrive in the presence of oxygen – even small traces of oxygen can kill these bacteria in a short time. Unlike aerobes, they cannot eliminate toxic oxygen radicals (aerobic bacteria have special enzymes such as catalase for this purpose). Obligate anaerobic bacteria obtain their necessary energy either by fermentation or by so-called anaerobic respiration.

Facultative anaerobic bacteria are tolerant as far as oxygen is concerned: they can grow both with and without oxygen. When oxygen is present, they obtain the energy they need through “normal” (aerobic) cellular respiration, just as aerobic bacteria and animal and human cells do. In an oxygen-free environment, on the other hand, their energy production proceeds via fermentation or anaerobic respiration.

Aerotolerant bacteria can thrive without problems in the presence of oxygen, but cannot use it for energy production.

Classification according to temperature requirements

Depending on the temperature range bacteria prefer or tolerate, three groups of bacteria are distinguished:

  • psychrophilic bacteria: They thrive best at five to ten degrees Celsius. The minimum temperature they can tolerate is -5 to -3 degrees, depending on the bacterial species, and their maximum temperature is 15 to 20 degrees, depending on the species.
  • Mesophilic bacteria: Their temperature optimum is 27 to 37 degrees. The temperature may drop to a maximum of 20 to 25 degrees. On the other hand, the temperature must not rise to more than 42 to 45 degrees.
  • Thermophilic bacteria: They feel most comfortable 50 to 60 degrees. Depending on the type of bacteria, the temperature must not drop below 40 to 49 degrees and must not rise to more than 60 to 100 degrees.

Classification according to taxonomy

Like other living organisms, bacteria are classified according to scientific criteria into different hierarchical levels such as families, genera and species. Some species of bacteria can also be further divided into different types (bacterial strains) – depending on hereditary factors and chemical composition.

How do bacteria reproduce?

Bacteria reproduce asexually by cell division:

How fast bacteria can multiply depends on the type of bacteria and the environmental conditions. Under optimal conditions, many bacteria can double their number within just twenty minutes.

When we talk about bacterial growth, we mean the increase in bacterial cell count. It is determined as the number of cells per milliliter.

What diseases are caused by bacteria?

There are a variety of diseases caused by bacteria. Here is a small selection:

  • Scarlet fever: This very contagious bacterial infectious disease is caused by the gram-positive, spherical A streptococci (Streptococcus pyogenes).
  • Other streptococcal infections: A streptococci can also cause otitis media, tonsillitis, erysipelas, pneumonia, and rheumatic fever, among others. B-streptococci (S. agalactiae) are possible triggers of meningitis and wound infections, for example. Other streptococci can occur, for example, as caries bacteria.
  • Pneumococcal infections: Pneumococci are also streptococci that usually occur as pairs (diplococci). More specifically, they are Streptococcus pneumoniae. This bacterium is a typical pathogen of pneumonia, but can also cause meningitis, middle ear or sinusitis, among other things.
  • Meningococcal infections: Meningococci are bacteria of the species Neisseria meningitis. Infections with these germs usually manifest themselves in the form of meningitis or bacterial “blood poisoning” (sepsis).
  • Gonorrhea (gonorrhea): This STD is also caused by Neisseria bacteria, this time Neisseria gonorrhoeae (also called gonococcus). Treated in time, gonorrhea usually heals without consequences. Otherwise, there is a risk of permanent late effects such as infertility.
  • Chlamydia infections: There are various types of chlamydia (some with subgroups) that can cause different clinical pictures, for example conjunctivitis, infections of the urinary and genital organs (such as urethritis, cervicitis or prostatitis) and pneumonia.
  • Whooping cough: The Gram-negative bacterium Bordetella pertussis is usually behind this “children’s disease”, which is also increasingly occurring in adolescents and adults.
  • Diphtheria: Symptoms such as a barking cough, difficulty swallowing, and sweetish foul breath are caused by the toxin of the gram-positive rod-shaped bacterium Corynebacterium diphtheriae.
  • Tuberculosis: Mycobacterium tuberculosis is the most common cause of this serious, notifiable infectious disease.
  • E. coli infections: Escherichia coli is a gram-negative bacterium of which there are several strains. Some of them live naturally in the intestines of healthy people. Other strains of E.coli, however, can cause infections, for example in the digestive or urinary tract (such as diarrhea and cystitis).
  • Salmonellosis (salmonella poisoning): This term refers to infectious diseases and food poisoning caused by a specific subgroup of Salmonella bacteria. It includes typhoid and paratyphoid fever, among others.
  • Listeria infection (listeriosis): This food poisoning is caused by gram-positive bacteria of the species Listeria monocytogenes. It is accompanied by nausea, vomiting and diarrhea. It can be contracted by eating contaminated foods such as dairy products, raw vegetables, or inadequately heated meat.
  • Cholera: The Gram-negative bacterium Vibrio cholerae is responsible for the severe diarrheal disease, which occurs mainly in areas with poor hygienic conditions.

Bacteremia and sepsis

Normally, bacteria are not found in the blood. If there are, it is called bacteremia. It can occur, for example, when someone gets bleeding gums from vigorous tooth brushing or cuts themselves with a pocket knife. Bacteria can also enter the bloodstream during bacterial infections (such as bacterial pneumonia) or during a dental or medical procedure.

Bacteremia does not always cause symptoms if the immune system quickly eliminates the bacteria.

Especially in people with a weakened immune system, however, the bacteria can cause an infection (e.g. inflammation of the inner lining of the heart = endocarditis) if they remain in the blood for a sufficiently long time and in larger numbers. The consequence can be a very violent reaction of the whole body, which is called sepsis (“blood poisoning”). In the worst case, it can lead to death. However, the risk of death in individual cases varies considerably. It depends, among other things, on the type of bacteria involved and how quickly the patient is treated.

Bacteria: Transmission or infection

For example, people can become infected with salmonella via smear infection: If people with salmonella-related diarrhea do not wash their hands thoroughly after going to the toilet, they can transfer the germs to objects (such as doorknobs, cutlery). If a healthy person touches these objects and then grabs their mouth, nose or eyes, they can become infected. Direct person-to-person infection through smear infection is also possible when infected persons shake hands with a healthy person with contaminated hands.

However, salmonella is mainly transmitted via contaminated food. This route of infection also exists for some other bacteria such as Listeria (causative agent of listeriosis) and representatives of the Campylobacter genus (causative agent of contagious diarrheal diseases).

The latter, like Salmonella and some other bacteria, can also be transmitted via contaminated water.

In some cases, infection is possible through sexual intercourse, as in the case of chlamydia and the causative agent of gonorrhea (gonococci).

Bacterial infection: Treatment

Some antibiotics are effective against many different types of bacteria (broad-spectrum or broad-spectrum antibiotics), while others target specific groups of bacteria (narrow-spectrum or narrow-spectrum antibiotics).

Well-known groups of antibiotics include penicillins, cephalosporins, tetracyclines, and macrolide antibiotics.

Not every bacterial infection requires treatment with antibiotics. Alternatively or in addition, other measures may be useful that do not specifically target bacteria, but at least relieve symptoms (e.g., pain and anti-inflammatory medications).

Vaccination against bacteria

Some infectious diseases caused by bacteria can be prevented with vaccination. The vaccine administered stimulates the immune system to develop specific antibodies against the bacterial pathogen in question (active immunization). This arms the immune system in case a “real” infection with these bacteria occurs later. The infection can thus be nipped in the bud at an early stage or at least weakened.

Examples of available vaccinations against bacteria:

  • Diphtheria vaccination
  • Whooping cough vaccination
  • Tetanus vaccination (also available as passive immunization, in which ready-made antibodies are injected)
  • Haemophilus influenzae type b vaccination (HiB vaccination)
  • Meningococcal vaccination
  • Cholera vaccination
  • Typhoid vaccination

Some of these vaccines are available as combination preparations of different compositions. For example, the Td vaccine protects against tetanus and diphtheria bacteria simultaneously.