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1. Classification

a. Higher order taxa

Bacteria (Eubacteria); Bacillota; Clostridia; Eubacteriales; Peptococcaceae; Peptococcus

Species

NCBI: [1]

“Peptococcus niger”

2. Description and significance

‘’Peptococcus niger’’ is a Gram-positive, anaerobic bacterium commonly found in the human body, particularly in the intestinal tract and gut flora [2]. This bacterium plays an important role in human digestion by recycling nutrients and breaking down organic compounds [3]. Additionally, sulfatase activity of ‘’P. niger’’ can contribute to the digestion of bile acid and steroids, which impact hormonal regulation and metabolic activity in humans [4]. Although this organism is not known to be pathogenic, in some cases, it can cause eye infections in immunocompromised individuals [5].

A distinctive aspect of ‘’Peptococcus niger’’ is its high % G+C content of its genome, which has ultimately led to it being the only remaining species of its genus. This genetic trait might offer a survival advantage to ‘’Peptococcus niger’’ by allowing it to thrive in challenging environments (3).

3. Genome structure

The whole genome of ‘’Peptococcus niger’’ consists of 1,965,692 base pairs and 1,916 genes, of which 1,823 are coding genes [6]. The bacterium's GC content is approximately 49.5% [7]. Some major groups of unique proteins encoded by ‘’Peptococcus niger’’ include sulfatase enzymes, which function to metabolize steroids [4] and bile acids [8]. The expression of these protein coding sequences play a vital role in impacting human steroid metabolism and disease progression [8].

4. Cell structure

The first ‘’Peptococcus niger’’ isolate, identified in 1930, formed smooth, round, black colonies about 0.5 mm in diameter. However, when exposed to oxygen, these colonies faded to a gray color [9]. It was soon discovered that when ‘’P. niger’’ is isolated on blood agar, it remains the same size and shape on the plate, with the black pigment persisting for several days without fading to a dull color [9].

According to a distance matrix analysis (a method used to compare genetic similarities), ‘’P. niger’’ is a Gram-positive bacteria while having distinct cell wall characteristics related to Gram-negative bacteria [3]. ‘’P. niger’s’’ black appearance and characteristics that bridge Gram-positive and Gram-negative bacteria has contributed to its classification as the only species in its genus. Although other related species have been transferred to the genus ‘’Peptostreptococcus, P. niger’’ still shares some traits with these reclassified species [10]. ‘’P. niger’’ is a non-spore forming, non-motile bacteria [10]. Because of its presence in human intestinal flora, this bacterium does not endure high oxygen exposure, nor does it require flagella for movement, as it can rely on diffusion for transport [11].

5. Metabolic processes

As an obligate anaerobe, ‘’P. niger’’ thrives in environments with little to no oxygen present [9]. Classified as a chemoorganotroph, the bacterium relies on breaking down organic molecules, such as proteins, fats and sugars for energy production and as carbon sources. Metabolically, ‘’P.niger’’ performs fermentation and anaerobic respiration to generate energy [4]. In bacterial vaginosis, when the normal vaginal bacterial balance is disturbed, this increases the growth of opportunistic anaerobic bacteria, such as ‘’P. niger’’, through fermentation [2].

‘’P.niger’’ contains wide range of sulfatase activities, by which they are able to desulfate steroid sulfates, including estrone-3-sulfate, into alcohol and bisulfate [11]. Through this process, the released sulfate can be utilized by ‘’P.niger’’ for biosynthesis. It has also been hypothesized that the released sulfur could serve as a potential electron acceptor for the bacterium. Additionally, ‘’P.niger’s’’ desulfation of aryl and alkyl steroid sulfates can impact the bioavailability and the steroid pool in a host, having hypothesized influence on host metabolic processes [11].

6. Ecology

‘’Peptococcus niger’’ belongs to the family Peptococcaceae, which are predominantly anaerobic organisms living in oxygen deprived environments, such as the gastrointestinal tract of humans around the world [11]. These microorganisms play a crucial role in the decomposition of organic matter by using fermentation to metabolize fatty acids and amino acids [6]. These metabolic processes are crucial for recycling and breaking down organic matter and releasing nutrients, such as sulfate esters, into the surrounding environment. Although ‘’Peptococcus niger’’ is specifically found in the gut microbiome of humans, in rare cases the bacterium has been found in mammals with weakened immune systems that provides a suitable environment for the bacteria to grow in [5]. Its presence in the human gut aids in digestion and maintaining a balance of the microbial flora in the gut. The gut provides an optimal growth environment for ‘’P. niger’’, being oxygen-deprived and a temperature ranging between 35-37℃ [12]. Optimal growth is also favored in neutral pH environments that range from 7.0 to 7.5[12].

7. Pathology

‘’P. niger’’ is not considered pathogenic to humans as it grows too slowly to cause disease [9]. However, it has been linked to infections in immunocompromised individuals [5]. Additionally, researchers have injected different regions of the body with ‘’Peptococcus niger’’, such as in the human vagina and found that this organism does not normally elicit any innate immune responses, such as inflammation, in the organs [13]. The exact mechanisms by which ‘’P. niger’’ causes diseases are not fully understood. Unlike other well-studied bacteria, it does not have specific toxins or surface proteins that usually lead to illness [9].

‘’P.niger’s’’ ability to grow in anaerobic conditions and break down a variety of organic materials may contribute to its survival and potential opportunistic behaviors that lead to infection [11].

8. Medical/Clinical Significance

‘’Peptococcus niger’’ has clinical and medical significance due to its potential role as an opportunistic pathogen, as well as in its metabolism of organic compounds. While it is still relatively rare in clinical practice, understanding the effects of ‘’P. niger’’ on immunocompromised individuals can potentially lead to the development of treatments for patients who suffer from bacterial pathogenicity [5].

One study discovered that Bixa orellana leaf extract contains antimicrobial properties to which the ‘’Peptococcus niger’’ and other organisms are sensitive to, making it a possible treatment for bacterial vaginosis [2]. Researchers can detect and distinguish between strains of ‘’P. niger’’ in mammals using PCR-RFLP (Polymerase Chain Reaction-Restriction Fragment Length Polymorphism) [14].

9. Current Research

A major area of current research on ‘’Peptococcus niger’’ has focused on the pathogenic potential of the bacterium. A study published in 2022 investigated the pathogenic potential of ‘’P. niger’’ in prostate tissues and the structural changes that occurred in those tissues due to infection [13]. There were acute inflammatory reactions that were observed in the samples of prostate tissue that were removed following bacterial contamination. The study was conducted using New Zealand rabbits as animal models, which means that further research is needed on the link between ‘’P. niger’’ and prostate inflammation in humans.

Another area of recent research has focused on the effectiveness of plant-based antimicrobial agents to combat anaerobic bacteria associated with bacterial vaginosis, including ‘’P. niger’’. In one study, it was found that ‘’P. niger’’, was susceptible to extract from Bixa orellana L. leaves [2]. These findings suggest the possibility that non-synthetic therapeutic alternatives could be used to treat bacterial vaginosis to target microorganisms such as ‘’P. niger.’’