Generality
The classification of bacterial species in aerobic and anaerobic bacteria is performed according to the energy source used to feed the biosynthetic processes of their metabolism.
Based on this type of classification, the various bacterial species can be divided into four large groups.
Obligatory aerobes
The bacteria belonging to this group draw energy from aerobic respiration ; therefore, they absolutely need oxygen (O 2 ) to survive.
Obligatory anaerobes
Obligatory anaerobes - also known as aerophobes - are bacteria that do not need O 2 to survive, but, on the contrary, the presence of oxygen in their habitat inhibits their growth.
These bacteria draw energy from other metabolic processes, such as fermentation, anaerobic respiration, bacterial photosynthesis or methanogenesis .
Optional aerobes / anaerobes
Oxygen is not indispensable for the growth of bacteria belonging to this group, but when it is available, it is still used.
These microorganisms, in fact, under anaerobic conditions (absence of O 2 ) draw energy from processes such as fermentation or anaerobic respiration, while in the presence of oxygen (aerobic conditions) they produce energy through aerobic respiration .
Aerotolerant anaerobes
These bacteria produce energy exclusively through anaerobic processes (generally, through fermentation ), but - contrary to what happens for the obligate anaerobes - the presence of oxygen in their habitat does not inhibit their growth.
The type of behavior that a bacteria has towards oxygen depends on the type of enzymes that the bacterium itself possesses. More specifically, it depends on enzymes capable of interacting with oxygen and its radicals, such as superoxide anion (O 2 -), hydroxyl radicals (• OH) and hydrogen peroxide (H 2 O 2 ) .
These enzymes are:
- Superoxide dismutase, is an enzyme present in obligate aerobes, in aerotolerant anaerobes and in many optional aerobes. Its job is to prevent the build-up of superoxide which would otherwise be lethal to the cell.
- Catalase, is an enzyme present in obligate aerobes and in many optional aerobes. Its task is to decompose the hydrogen peroxide (H 2 O 2 ) into water (H 2 O) and oxygen (O 2 ). In this way, the enzyme detoxifies the bacterial cell from this extremely harmful component.
- Peroxidase, is an enzyme present in many aerotolerant anaerobes. Its task is to convert hydrogen peroxide (H 2 O 2 ) into water (H 2 O) through an electron transfer mechanism.
The obligate anaerobes do not possess these enzymes, which is why the presence of oxygen in their habitat inhibits their growth.
Oxygen radicals are formed as waste products of metabolic processes, both of prokaryotic cells (such as bacterial ones) and of eukaryotic cells (such as animal and plant cells). These radicals are harmful to the cell and must be immediately degraded to prevent them from exerting their toxic action. Therefore, only the bacteria that possess the enzymes responsible for the degradation of oxygen radicals can survive and grow in aerobic conditions.
Photosynthetic organisms (such as plants, algae and some bacterial species) and even some non-photosynthetic organisms, are able to protect themselves from the action of radicals of O 2 thanks to the presence of particular pigments, carotenoids . These pigments, in fact, are able to neutralize toxic oxygen radicals, protecting the cell from oxidative mechanisms.
Aerobic and pathogenic anaerobic bacteria
Some of the main aerobic and anaerobic bacteria that can be pathogenic for humans will be briefly described below.
Pathogens obliged to humans
There are many bacterial species that are pathogenic to humans belonging to this group, among which we recall:
- Bordatella pertussis, responsible for infections of the respiratory tract that cause pertussis or acute bronchitis. The first choice treatment against B. pertussis infections involves the use of erythromycin (a macrolide); alternatively, amoxicillin can be used. A vaccine is also available to prevent this type of infection.
- Legionella pneumophila, responsible for the onset of Legionnaires' disease (or Legionnaire's disease). Legionellosis can be treated with drugs such as azithromycin, erythromycin, clarithromycin, telithromycin or fluoroquinolones.
- Mycobacterium leprae, is responsible for the onset of leprosy. To counter infections caused by this micro-organism, drug associations are used such as dapsone and rifampicin or acedapsone and clofazimine.
- Neisseria gonorrhoeae, is responsible for the onset of pharyngeal gonorrhea, gonorrhea, acute gonococcal urethritis, prostatitis, cervicitis, endometritis, pelvic inflammatory disease, dermatitis and arthritis. Cephalosporins (such as ceftriaxone and cefixime) or fluoroquinolones are usually used to treat N. gonorrhoeae infections.
Pathogen obligate anaerobes for humans
Among the various pathogenic bacteria for humans that can grow only in habitats without oxygen - therefore in anaerobic conditions - we recall:
- Clostridium difficile, this bacteria can be part of the normal human bacterial flora and is responsible for opportunistic infections of the gastrointestinal tract. Antibiotics such as metronidazole, chloramphenicol, vancomycin or erythromycin are usually used against C. difficile infections.
- Clostridium tetani, responsible for tetanus (or spastic paralysis). Generally, metronidazole or benzylpenicillin are used against this bacteria. A vaccine is also available to prevent infection.
- Clostridium botulinum, responsible for botulism (or flaccid paralysis).
- Bacterioides fragilis, is responsible for the onset of abdominal abscesses, appendicitis, peritonitis, rectal abscesses or septicemia. Antibiotics such as metronidazole, clindamycin or carbapenems are used to treat B. fragilis infections.
Optional pathogenic human aerobes / anaerobes
Among the optional aerobes / anaerobes that may be pathogenic to humans, we recall:
- Escherichia coli, is a bacteria normally present in human intestinal flora, but in immunosuppressed subjects it can give rise to opportunistic infections that can cause urethrocystitis, prostatitis, neonatal meningitis, enterohaemorrhagic colitis, watery diarrhea or traveler's diarrhea.
The most widely used antibiotics to treat E. coli infections are carbapenems, penicillins, monobactams, aminoglycosides, cephalosporins or macrolides.
- Haemophilus influenzae, is responsible for infections of the respiratory tract and the nervous system. The antibiotics usually used to fight infections caused by this type of infection are cephalosporins, penicillins or sulfa drugs.
Aerotolerant pathogenic anaerobes for humans
Among the various pathogenic bacteria belonging to this group, we mention Propionibacterium acnes and Propionibacterium propionicum .
P. acnes is part of the normal bacterial flora present on human skin and is involved in the development of acne. In some cases, however, P. acnes can cause opportunistic infections that lead to the onset of meningitis, endocarditis, arthritis or surgical infections.
P. propionicum is instead responsible for the onset of lacrimal canaliculitis and dental abscesses.
To treat infections caused by this type of bacteria, drugs such as penicillins, cephalosporins, quinolones or vancomycin can be used.
Prevention of bacterial infections
Bacteria are microorganisms found everywhere, with which we come into contact daily. Some of these microorganisms are harmless to our body, some are useful, while others can be very dangerous.
Generally, a few simple precautions are sufficient to prevent bacterial infections, such as, for example:
- Wash your hands thoroughly and thoroughly before starting to cook, eat or touch objects that may be a vehicle for bacterial spread;
- Follow the basic hygiene rules during food storage, preparation or cooking.
However, simple compliance with hygiene standards is not always sufficient to prevent bacterial infections.
To prevent sexually transmitted bacterial infections, for example, it is essential to use barrier contraceptive methods (such as, for example, condoms).
For the prevention of some types of bacterial infections, sometimes potentially fatal, vaccines are also available (an example may be tetanus vaccination).
Finally, the role of each individual's immune system must also not be underestimated. In fact, a functioning immune system can sometimes be sufficient to prevent the development of many bacterial infections.
