ANAEROBIC BACTERIA: CHARACTERISTICS, TYPES, SPECIES, CULTURE - BIOLOGY - 2024

The anaerobic bacteria are those that do not use oxygen in their metabolic processes. In fact, many of these bacteria cannot survive in environments where this element is present, as it is toxic to them.

One of the metabolic processes carried out by this type of bacteria and that has been most widely studied is fermentation. This is a process that has a large number of applications at the industrial level, related to the production of alcoholic beverages, yogurt and vinegar, among other products.

Bacteroides, a genus of anaerobic bacteria. Source: US gov

Similarly, many of these bacteria have their natural habitat within the human body, being found mainly in the gastrointestinal tract. Some are harmless, but others cause various pathologies in the body, many of which can lead to death.

characteristics

Anaerobic bacteria are among the first living things to emerge on the planet. Importantly, thousands of years ago, when the first life forms began to emerge, oxygen levels in the atmosphere were really low. Because of this, the first organisms that were developed had other cellular mechanisms to obtain energy.

In fact, for many of the species that make up the group of anaerobic bacteria, oxygen is highly toxic. This is why some bacteria are found in habitats where there is no oxygen present.

Likewise, there are anaerobic bacteria that support living in the presence of very small amounts of oxygen, so they can be located in a greater variety of environments. Some of these bacteria are part of the regular human microbiota, specifically at the level of the oral cavity and the gastrointestinal tract.

On the other hand, these bacteria carry out fermentation. This is a catabolic process by which organic compounds are degraded to simpler ones in the absence of oxygen. It is carried out mainly by anaerobic bacteria.

Likewise, there is no single type of fermentation. Depending on the product obtained there are several types, such as acetic, lactic and alcoholic fermentation, among others.

In these bacteria the various enzyme systems function perfectly in the absence of oxygen. This is because they use other compounds such as fumarate, nitrate or sulfate as electron acceptors.

It is appropriate to mention that many of the anaerobic bacteria constitute human pathogens. Among the best known and studied, the species of the genera Porphyromonas, Clostridium and Prevotella, among others, can be mentioned.

Types of anaerobic bacteria

Many of the anaerobic bacteria cannot resist being in environments where there is oxygen, because this is toxic for them. However, there are also some, despite not using oxygen, they can survive in environments where there are different concentrations of this gas.

With this in mind, anaerobic bacteria can be divided into three types: obligate anaerobic, aerotolerant, and facultative anaerobic.

Obligated anaerobes

They are also known as strict anaerobic bacteria. They are those that do not use oxygen for any metabolic process. Your tolerance to this gas is variable. According to this, obligate anaerobic bacteria are classified into:

- Strict: tolerate only oxygen concentrations below 0.5%.

- Moderate: they can tolerate oxygen concentrations ranging between 2 and 8%.

Aerotolerant anaerobes

These are bacteria that, although they do not use oxygen to carry out any cellular process, can live in habitats where it is present. A classic example of this type of bacteria is Pronibacterium acnespio, which causes many cases of acne.

Facultative anaerobes

Facultative anaerobic bacteria do not use oxygen for their metabolic processes, but they can live in the presence of it, since it is not toxic to them. They mainly carry out the fermentation process to obtain the necessary energy. Among the best known facultative anaerobic bacteria are Escherichia coli and Staphylococcus aureus.

Anaerobic bacteria species

To facilitate the study of anaerobic bacteria, they can be grouped according to their shape and staining into: gram-negative cocci, gram-positive cocci, gram-negative rods, and gram-positive rods.

-Gram negative cocos

Within this group are several genera such as Veillonella and Megasphaera.

Veillonella

This is a genus of gram negative bacteria that is well known for its fermentative potential. These bacteria ferment lactate to acetate and propionate through a metabolic pathway known as the methylmalonyl-CoA pathway.

Likewise, they are an integral part of the microbiota of certain parts of the body such as the oral cavity and the gastrointestinal tract.

-Gram positive cocos

They are round shaped bacteria that, when stained with the gram pigment, adopt a characteristic violet color.

Anaerococcus

Bacteria of this genus are found forming part of the bacterial flora of the human body. However, it is possible that it is the cause of certain pathological processes. This bacterium has been isolated from vaginal discharge, ovarian abscesses, and certain chronic wounds.

Likewise, in different clinical case studies it has been established that species of this genus can cause a wide range of infections in the urinary tract, as well as in the blood.

Pepto-streptococcus

It is a genus of bacteria that frequently cause the development of pathologies in humans. In addition, they are part of the microbiota of certain mucosa of the body, such as the oropharyngeal and genitourinary.

-Gram negative bacilli

They are bacteria that have an elongated rod shape and are characterized by acquiring a fuchsia color when subjected to the Gram staining process. This group of anaerobic bacteria encompasses several genera.

Prevotella

It is a genus of bacteria that are frequently found as components of the bacterial flora of the oral cavity, the gastrointestinal tract, and the vaginal cavity. Despite this, when normal body conditions change, these bacteria proliferate uncontrollably.

When this occurs, Prevotella causes the development of certain infections, the most common being those that occur at the level of the periodontal mucosa. They have also been isolated from patients with rheumatoid arthritis.

-Gram positive bacilli

To this group belong rod-shaped bacteria that have a thick layer of peptidoglycan in their cell wall that retains the gram dye particles, giving it a violet coloration. This group includes a wide variety of species that cause various diseases and pathologies in humans.

Clostridium botulinum

It is a gram positive bacterium that is characterized by producing spores, which constitute an infectious agent for humans. In the same way, this bacterium synthesizes and releases certain toxins, whose main function is to paralyze the muscles.

Among the diseases that this bacterium generates, we can mention food botulism, which affects the gastrointestinal tract, and infant botulism, which affects young children and is caused by the direct ingestion of spores.

Clostridium tetani

This bacterium is well known in the field of medicine and has been the subject of numerous studies on the effects it generates in the body. Like the rest of the bacteria of the genus Clostridium, this bacterium produces endospores, which are very resistant to environmental components.

Clostridium tetani. Source: Content Providers (s): CDC

Likewise, this bacterium causes a pathology in humans known as tetanus. This disease is caused by the entry of spores into the body, which germinate and begin to synthesize and release the toxins of this species.

When this occurs, the spores interfere with the transmission of nerve impulses, causing the muscles to spasm painfully.

Differences from aerobic bacteria

The main difference between anaerobic bacteria and aerobic bacteria refers to the use of oxygen and the ability to develop in the presence or absence of this element.

In this sense, anaerobic bacteria do not use oxygen for any of their internal energy production processes. On the contrary, aerobic bacteria do use it as an electron acceptor during the process of cellular respiration, by means of which they degrade molecules of organic compounds such as glucose, obtaining a large amount of energy.

Likewise, there are anaerobic bacteria that, apart from not using oxygen in their various processes, cannot be in oxygen-rich environments, since this element is highly toxic to them. Instead of cellular respiration, they run fermentation.

Another notable difference between the two types of cells has to do with their habitat. Aerobic bacteria are found in environments where oxygen is widely available, whereas aneroid bacteria survive in environments where there is a complete absence of oxygen or where oxygen concentrations are very low.

Anaerobic bacteria culture

The cultivation of anaerobic bacteria is a process in which all the necessary precautions must be followed to avoid making a mistake. Here it is important to bear in mind that some species of anaerobic bacteria cannot have any type of contact with oxygen, since this can be a toxic agent for them.

One of the elements to take into account when carrying out a culture in the laboratory is the culture medium that should be used. Each group of bacteria has certain specific requirements.

Culture media

At present there are many culture media that can be used for the sowing of any type of bacteria. In the specific case of anaerobic bacteria, the culture medium requires the addition of a reducing agent, such as thioglycollate or cysteine.

Likewise, to optimize the results that are expected to be obtained, a redox indicator can be added, which shows the possible presence of oxygen, such as resazurin.

The most widely used and recommended culture media for seeding anaerobic bacteria are:

- Blood agar for anerobios: contains, among other things, meat extract, yeast extract, NaCl, glucose and peptone, among others.

- Chocolate agar: it is not widely used to cultivate anaerobic bacteria. However, thanks to its components it is possible to make them grow there.

- Ethyl phenyl alcohol agar with blood: this medium is widely used to properly isolate anaerobic bacteria.

- Columbia agar with colistin and nalidixic acid.

- Thioglycollate: it is generally enriched with L-cystine, hemin and vitamin K.

- KVLB agar (kanamycin, vancomycin, lacquered blood): specifically for gram negative bacilli.

Process

Once the culture medium to be used has been selected, according to the bacteria to be cultivated, the procedure can begin. In this sense, a totally anaerobic environment must be created, in which there is no oxygen. To achieve this, two procedures can be followed:

- Sodium hydroxide - pyrogallic acid method: this method is based on obtaining an anaerobic environment through a chemical reaction, using, instead of a reducing agent, a catalyst. It is a delicate and potentially dangerous procedure.

- Use of anaerobic flasks: this type of flasks are very useful for the cultivation of anaerobic organisms. However, it is only used with organisms that can withstand light contact with small levels of oxygen.

Likewise, a kind of vacuum must be created to proceed with cultivation. This can be done in a number of ways: displacing oxygen with other gases, absorbing oxygen through chemical or biological methods, or using reducing agents.

Culture of Clostridium perfringens. Source: Photo Credit: Content Providers (s): CDC / Dr. Stuart E. Starr

Once these steps have been carried out, inoculation is carried out to incubate the culture and wait the reasonable time to study the colonies obtained. The waiting time will depend on the species that is being cultivated.

There are fast growing species (18-24 hours) such as Clostridium perfringens and there are others that have slow growth (5-7 days) such as Propionibacterium.

Methods for plate counting

These methods are used to find out the approximate number of microorganisms found in a culture. In the case of anaerobic bacteria, there are several techniques that allow counting.

However, since many of these bacteria cannot come into direct contact with oxygen because it is toxic, extreme care must be taken when practicing these techniques.

In this sense, two methods will be described here: the most probable number (for strict aerobics) and the plaque spreading method (for facultative and microaerophilic anaerobes).

Most likely number method

This technique is based on the determination of the presence or absence of microorganisms in a sample. This is done in replicates of consecutive solutions. The principle behind this method is that a single living cell can develop, thus producing a cloudy culture.

To carry out this process, it is necessary to make a series of serial dissolutions of the culture sample. For this, it is important to use a liquid culture medium with the nutritional requirements. Once this is done, the test tubes are incubated while waiting for the microorganisms to grow.

After the reasonable time has elapsed, the test tubes into which one or more bacterial cells were inoculated will appear cloudy.

As the tubes in which the dissolution factor is increasing are reviewed, they become progressively less cloudy, as they contain fewer and fewer bacterial cells. This is the case until a point is reached where there will no longer be any or only one microorganism.

Finally, the population density estimate is obtained from the pattern of occurrence in serial solutions and through the use of a standard probabilistic table that has been designed for this purpose.

Plate spreading method

This is a fairly straightforward method. First, several Petri dishes with the culture medium should be arranged in an orderly manner. Subsequently, 1 mL of solution is inoculated into each capsule. It is important to note that the solutions contain the microorganism to be counted.

Then, with the help of a glass rod, the inoculated material is spread homogeneously covering the entire surface of the culture medium.

Finally, the Petri dishes are incubated under the conditions required for the time necessary for each type of microorganism. Finally, the colonies that have been generated are counted.

Diseases caused

Many of the anaerobic bacteria are known pathogens for man and cause diseases and pathologies that can even compromise the life of the individual.

Infections at the level of the oral cavity

The oral cavity is the ideal environment for many anaerobic bacteria, which proliferate there and generate certain pathologies such as periodontitis, necrotizing gingivitis and abscesses, among others.

Most of these infections present with pain, lesions in the mucosa and suppuration of pus, with the consequent increase in body temperature.

Among the causative bacteria, Porphyromonas gingivalis is the one most frequently isolated in these lesions.

Intestinal infections

One of the routes of infection for this type of bacteria is through ingestion, either spores (in the case of sporulated bacteria) or contaminated food and water.

In these cases, the bacteria anchor to the intestine and begin to cause damage to the intestinal mucosa, triggering symptoms such as: fever, abdominal pain, liquid stools, bloating, nausea and general malaise.

The anaerobic bacteria that can generate this clinical picture are Clostridium difficile and Escherichia coli, among others.

Skin infections

Some of these bacteria, when they come into contact with an open wound, can infect it. If this occurs, severe skin infections can be generated, such as gas gangrene, whose causative agent is Clostridium perfringes.

Symptoms of gas gangrene include: blisters filled with bloody fluid, subcutaneous emphysema, severe pain and high fever, among others.

Another pathology caused by anaerobic bacteria at the cutaneous level is necrotizing fasciitis, in which the infection spreads beyond the skin tissue, even affecting the fascia that surrounds the muscles.

Symptoms include acute pain, severe erythema, blisters, and most often gangrene. Among the causative agents of this pathology is Staphylococcus aureus.

References

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