Infections caused by anaerobic bacteria are common, and may be serious and life-threatening. Anaerobes predominant in the bacterial flora of normal human skin and mucous membranes, and are a common cause of bacterial infections of endogenous origin. Infections due to anaerobes can evolve all body systems and sites. The predominate ones include: abdominal, pelvic, respiratory, and skin and soft tissues infections. Because of their fastidious nature, they are difficult to isolate and are often overlooked. Failure to direct therapy against these organisms often leads to clinical failures. Their isolation requires appropriate methods of collection, transportation and cultivation of specimens. Treatment of anaerobic bacterial infection is complicated by the slow growth of these organisms, which makes diagnosis in the laboratory only possible after several days, by their often polymicrobial nature and by the growing resistance of anaerobic bacteria to antimicrobial agents.

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Anaerobes as Normal Flora and source of bioactive compounds

ANAEROBES AS NORMAL FLORA AND SOURCE OF BIOACTIVE COMPOUNDS



Composition of normal flora

The human body mucosal and epithelial surfaces are colonized with aerobic and anaerobic micro-organisms.Anaerobes outnumber aerobic bacteria on all mucosal surfaces, and certain organisms predominate in the different sites (Table 1). Differences in the environment, such as oxygen tension, pH and variations in the ability of bacteria to adhere to these surfaces, account for changing patterns of colonization. Microflora also varies in different sites within the body. In the oral cavity the organisms in the buccal folds vary in their concentration and strains types from those isolated from the tongue or gingival sulci. The prevailing organisms in one body system tend to belong to certain major bacterial species. The relative and total counts of organisms can be affected by various factors, such as age, diet, anatomic variations, illness, hospitalization and antimicrobial therapy. However, these predictable pattern of bacterial flora remain stable through life.

Recognizing the flora at certain sites is useful for predicting which organisms may be involved in an infection adjacent to that location and can assist in the selection of empiric antimicrobial therapy, even before the exact microbial etiology is known. The clinical microbiology laboratory considers the site`s normal flora and chooses selective culture media that interferes with the growth of certain bacteria.

The anaerobic skin flora is made up of Propionibacterium2 ( mostly Propionibacterium acnes ) and Peptostreptococcus spp. The perineum and lower extremity are colonized by colonic and vaginal flora.

The upper airways flora (oral, nasopharynx and oropharynx) is complex and contains many types of anaerobes. 1 Their distribution within the mouth is a function of their adherence and susceptibility to oxygen. The ratio of anaerobic bacteria to aerobic bacteria in saliva is approximately 10:1. The total count of anaerobic bacteria in the saliva and elsewhere in the oral cavity reach 107–108 bacteria/ml. The predominant anaerobes in the upper airways include Fusobacterium spp. (especially Fusobacterium nucleatum), pigmented Prevotella and Porphyromonas spp., Prevotella oralis and Peptostreptococcus spp.They also predominate in oropharyngeal, otolaryngologic and pulmonary infections.

The gastrointestinal flora varies at different levels. The stomach acidity reduces the number of organisms swallowed from the oropharynx. The stomach, duodenum, jejunum and proximal ileum normally contain few bacteria. The bacterial counts in the small intestine are relatively low, with total counts of 102–105 organisms/ml. However, the flora becomes more complex, and the number of species increases in the distal portion of the gastrointestinal tract. Interruption in intestinal motility may result in an increase in the number of bacteria. The organisms that predominate up to the ileocecal valve are Gram-positive facultative, whereas below that structure BacteroidesPeptostreptococcus and Clostridium spp., and coliform bacteria predominate.3 The mean number of bacteria in the colon exceeds 1011 bacteria/g fecal material. Approximately 99.9% of these bacteria are anaerobic (ratio of aerobes to anaerobes is 1:1000–10,000). In the colon 300–400 different species or types of bacteria can be found.

  

                                          Indigenous microflora of the colon

The female genital flora is composed of mixed aerobic and anaerobic flora. However, the concentration and type of bacteria is less stable and can be influenced by antimicrobial therapy, pregnancy and gynecologic surgery. A concentration of 108 organisms/ml is present during the reproductive years. Changes occur in the number of organisms at the various stages of the menstrual cycle.The predominant aerobic organisms are lactobacillus, and the predominant anaerobic bacteria are anaerobic Lactobacillus, Peptostreptococcus, PrevotellaBacteroides spp. and Clostridium spp. Other anaerobic include Porphyromonas. FusobacteriumBilophilia, Bifidobacterium, Actinomyces,Eubacterium, and Propionibacterium sppEnterobacteriaceae can be found in postmenopausal flora. Bacterial vaginosis is associated with an increase in the number of anaerobic flora and a decrease in the concentration of lactobacilli.5



The role of bacterial flora in infections

Most anaerobic infections originate from the endogenous mucosal membrane and skin flora. Anaerobes belonging to the indigenous flora of the oral cavity can be recovered from adjacent infections, such as cervical lymphadenitis, subcutaneous abscesses and burns in proximity to the oral cavity, human and animal bites, paronychia, tonsillar and retropharyngeal abscesses, chronic sinusitis, chronic otitis media, periodontal abscess, thyroiditis, aspiration pneumonia, and bacteremia associated with one of the above infections.3 The predominant anaerobes in these infections are  Prevotella,PrevotellaPorphyromonasFusobacterium and Peptostreptococcus spp., which are all part of the normal flora of the mucous surfaces of the oropharynx (Tables 2,3).
A similar correlation exists in infections associated with the gastrointestinal tract. Such infections include peritonitis after rupture of a viscus, liver and spleen abscess, abscess and wounds near the anus, intra-abdominal abscess, and bacteremia associated with any of these.3 The predominate anaerobes are B. fragilis group, Clostridium spp. (including C. perfringens) and Peptostreptococcus spp.

Another site where a correlation exists between the normal flora and the anaerobic bacteria recovered from infected sites is the genitourinary tract. The infections involved are amnionitis, septic abortion and other pelvic inflammations.3 The recovered anaerobes are usually Gram-negative bacilli and Peptostreptococcus spp. Organisms belonging to the vaginal–cervical flora are also important pathogens of neonatal infections. They can be acquired by the newborn before delivery in the presence of amnionitis, or during passage through the birth canal.




Anaerobes as sources of health promoting bioactive compounds

Anaerobes possess diverse physiological functions and are capable of producing a range of bioactive compounds that can promote human health. Anaerobes play health-promoting roles through their bioactive products as well as by their whole cells. The bioactive compounds produced by these microorganisms include antimicrobial agents and substances such as immunomodulators and vitamins. Bacteriocins produced by anaerobes have been used as preservatives for decades. Because these substances are effective at low concentrations, encounter relatively less resistance from bacteria and are safe to use, there is a growing interest in their antimicrobial activity.

Several antimicrobials have been recovered from the cultures of anaerobes. Closthioamide and andrimid produced by Clostridium cellulolyticum and Pantoea agglomerans, respectively, are examples of novel antibiotics of anaerobic bacteria origin. As research expands more such agents may be discovered.

Anaerobes are widely used in preparation of fermented foods and beverages. During the fermentation processes, these micro-organisms produce a number of bioactive compounds including anticancer, antihypertensive and antioxidant substances. The health promoting effect of fermented food is mostly due to these bioactive compounds.

In addition to their products, whole cell anaerobes enhance the quality of life. Probiotic anaerobeic bacteria have been used for many years and are achieving growing acceptance as health promoters. Gatrointestinal anaerobes have been used to treat severe Clostridium difficile infection syndromes including diarrhea and colitis which cannot be treated by other modalities.

Whole cell anaerobes are also studied to detect and treat cancer. Growing evidence is emerging that anaerobic bacteria that are members of the human microbiome are linked to individual’s overall health. A dysfunctional microbiome may be the cause of many diseases including cancer, allergy, infection, obesity, diabetes and several other disorders. Maintaining normal endogenous microflora is believed to alleviate some of these life threatening health problems. The use of probiotics and prebiotics which favorably alter the number and composition of the gut microflora is known to render a health promoting effect.

Human’s interaction with the microbiome anaerobes is complex. The unraveling of the associations between the human microbiome and the human body, may bring a significant shift in the way these diseases are diagnosed and treated. Application of this knowledge can enhance human health and improve the quality of life.



  




REFERENCES
1. Hentges DJ.The anaerobic microflora of the human body. Clin Infect Dis. 1993 ;16 4:S175-80.
2. Brook I, Frazier EH. Infections caused by Propionibacterium species. Rev Infect Dis 1991;13:819–22. 
3. Finegold SM. Anaerobic infections in humans: an overview. Anaerobe 1995;1:3–9.
4. Bartlett JG, Polk BF. Normal vaginal flora in relation to vaginitis. Obstet Gynecol Clin N Am 1989;16:329–36.
5. Vallor AC, Antonio MA, Hawes SE, et al. Factors associated with acquisition of, or persistent colonization by, vaginal lactobacilli: role of hydrogen peroxide production. J Infect Dis. 2001;184:1431-6.
6. Mamo G. Anaerobes as Sources of Bioactive Compounds and Health Promoting Tools. Adv Biochem Eng Biotechnol. 2016 Jul 19.






Table 1:  Normal Flora




Predominant

No. of organisms/gr
anaerobic
Site
Aerobes
anaerobes
bacteria
skin


P. acnes



Peptostreptococcus sp.




mouth/upper respiratory tract
108-9
109-11
pigment Prevotella & Porphyromonas spp.



Fusobacterium spp.



Peptostreptococcus spp.



Actinomyces spp.
Gastrointestinal tract



   upper
102-5
103-7
(     B. fragilis group



(     Clostridium spp.
   lower
105-9
1010-12
(     Peptostreptococcus spp.



(     Bifidobacterium spp.



(     Eubacterium spp.
   female genital tract
108
109
Peptostreptococcus spp.



Prevotella bivia



Prevotella disiens









Table 2:  Recovery of anaerobic bacteria in infectious sites



Infection


Peptostrepto- coccus
 sp.


Clostridium 
sp.

Bacteroides
fragilis
group

Pigmented Prevotella
 and Porphyromonas


P. bivius
P. disien

Fuso-bacterium 
sp.







Bacteremia
1
1
2
1
0
1
Central nervous system
2
1
1
2
0
1
Head and neck
3
1
1
3
0
3
Thoracic
2
1
1
3
0
3
Abdominal
3
3
3
1
1
1
Obstetric-gynecology
3
2
1
1
2
1
Skin and soft tissue
2
1
2
2
1
1
Frequency of recovery in anaerobic infections:  0 = none, 1 = rare (1% to 33%), 2 = common (34% to 66%), 3 = very common (67% to 100%).



  

 Table 3: Aerobic and anaerobic bacteria isolated in type of infection 

Type of infection
aerobic and facultative organism
Anaerobic
Pleuro pulmonary
Streptococcus viridans
Pigmented Prevotella (P. denticola, P. melaninogenica, P. intermedia, P. nigrescens, P. loescheii)

Staphylococcus aureus*
Nonpigmented Prevotella (P. oris, P. buccae, P. oralis)

Enterobacteriaceae *
Fusobacterium nucleatum (subsp. nucleatumpolymorphum)

Pseudomonas aeruginosa
Peptostreptococcus (P. micros, P. anaerobius, P. magnus)


Bacteroides fragilis group


Non-spore-forming gram-positive rods (Actinomyces, Eubacterium, Lactobacillus)
Intra-abdominal
Escherichia coli
Streptococcus (viridans group and group D)
Pseudomonas aeruginosa
Bacteroides fragilis group
Bilophila wadsworthia
Peptostreptococcus (especially P. micros)
Clostridium spp.
Female genital tract
Streptococcus (groups A, B, others)
Escherichia coli
Klebsiella pneumoniae
Neisseria gonorrhea (in sexually active patients)
Chlamydia (in sexually active patients)
Mycoplasma hominis (in postpartum patients)
Peptostreptococcus spp.
Prevotella (especially P. bivia, P. disiens)
Bacteroides fragilis group
Clostridium (especially C. perfringens)
Actinomyces, Eubacterium (in intrauterine contraceptive device-associated infections)
Skin and Soft tissue
Staphylococcus aureus
Streptococcus (Str. milleri group, groups A, B viridans group)
Enterococcus spp.‡
Enterobacteriaceae
Pseudomonas aeruginosa*
Peptostreptococcus (P. magnus, P. micros, P. asaccharolyticus)
Pigmented Prevotella spp.
Actinomyces spp.
Fusobacterium nucleatum
B. fragilis group‡
Clostridium spp. ‡

* = recovered in hospital-acquired infection
‡ = better exposure to colonic flora.