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  Enterococcus faecium (week 13)

•  Bacteria colony and cell morphology

Figure 1: E. faecium facultative aerobe

Enterococcus faecium are Gram-positive bacteria with a coccus (spherical/ovoid) morphology usually appearing in chains or pairs. On a macroscopic level (solid media), E. faecium appear as white colonies (smooth and creamy) throughout the entire edge, each colony measuring 1-2mm (Health protecting agency, 2010). They are known to have thin growth characteristics with a convex elevation and opaque opacity. Also, on an FTM (Fluid Thioglycollate Medium) tube, due to its ability to make ATP when oxygen is present, and still manage to use fermentation when oxygen is not present making it a facultative aerobe (figure 1) They are motile bacteria (flagellum present) that have odors of a wet dog.

     

• Chemical and metabolic test results and interpretations.

Figure 2: Negative catalase test for E. faecium colony.

After discovering the bacteria was a Gram-positive cocci, streaking process took place on both MSA (Mannitol salt agar) and MAC (MacConkey agar) media plates. MSA is both a selective and differential media containing 7.5-10% salt concentration. It is a media that selects for Gram-positive bacteria and is differential for mannitol-fermenting staphylococci (Bachoon, 2008). MAC is also both a selective and a differential medium. It selects for Gram-negative bacteria, inhibiting Gram-positive bacteria. E. faecium grew well on MSA media. The catalase tests primary job is to detect the presence of the enzyme catalase, hence release of oxygen from hydrogen peroxide. E. faecium showed negative catalase presence (figure 2), which lead to the next test, detecting a hemolytic process. The detection of alpha, beta, and gamma hemolysis (Red Blood Cell destruction) is viewed on a Blood agar plate. E. faecium showed no hemolysis (alpha/gamma hemolysis). The next test performed was the bile esculin slant. The Bile Esculin Slant is used to isolate and identify bacteria that can hydrolyze esculin (differential constituent) when bile (selective ingredient) is present. The primary job of a bile esculin is to differentiate between Enterococcus and Streptococcus species. Hence, Enterococcus grow in the existence of bile (figure 3).

Figure 3: Bile esculin slant. growth (zigzag-ish) can be observed

(Grabsch, 2008). Mannitol fermentation is determined by two processes. One is the production of acid products due to mannitol fermentation, which alters the color of the medium. The second process is a follow up from the first one. When mannitol is fermented, the pH of the broth culture drops. The Enterococcus faecium displayed negative acidity.

• Kirby-Bauer test results

Figure 4: Kirby-Bauer test sensitivity E. faecium

A Kirby-Bauer test is a famous antibiotic testing method in which different antibiotics are used to kill or inhibit bacteria, depending upon their sensitivity. For a Kirby-Bauer test to take place, Two things must be prepared. One is a Mueller Hinton Agar plate, which contains starch that has characteristics of acting like a mixture inhibiting toxic materials (Haltiner, 1980). The second materials needed were the actual antibiotics in little discs (wafers). The Mueller Hinton agar along with the discs (antibiotics) perform the zone of inhibition. The zone of inhibition is an area around the disk in which bacteria are killed or stopped from growing, due to antibiotic presence. Four antibiotics were picked, Polymyxin B, Ampicillin, Neomycin, and ciprofloxacin (figure 4). As observed from (figure 4), the Enterococcus faecium is likely to be affected by the antibiotics.

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                                                                                               Course of treatment

Enterococcus faecium is known to be resistant (figure 4) for multiple antibiotics. For example, E. faecium is resistant to penicillin, vancomycin, streptomycin… The best way to treat E. faecium is to combine antibiotics like penicillin or ampicillin with the likes of ceftriaxone. Because treatments are different by location of E. faecium infections when skin is infected its best treat with daptomycin, linezolid, tedizolid… (Merck Manual, 2017).

References

Bachoon, Dave S.; Dustman, Wendy A. (2008). "Exercise 8: Selective and Differential Media for Isolation". In Michael Stranz. Microbiology Laboratory Manual. Mason, OH: Cengage Learning.

Enterococcal Infections - Infectious Diseases. (n.d.). Retrieved November 23, 2017, from https://www.merckmanuals.com/professional/infectious-diseases/gram-positive-cocci/enterococcal-infections

Grabsch EA, Ghaly-Derias S, Gao W, Howden BP (December 2008). "Comparative study of selective chromogenic (chromID VRE) and bile esculin agars for isolation and identification of vanB-containing vancomycin-resistant enterococci from feces and rectal swabs". J. Clin. Microbiol. 46 (12): 4034–6. doi:10.1128/JCM.00944-08. PMC 2593271 . PMID 18832121.

Haltiner, R.C., P.C. Migneault, and R.G. Roberston. 1980. Incidence of Thymidine-Dependent Enterococci Detected on Mueller-Hinton Agar with Low Thymidine Content. Anti. Ag. and Chemo.; 18(3): 365-368

Health Protection Agency (2007). Identification of Streptococcus species, Enterococcus species and morphologically similar organisms. National Standard Method BSOP ID 4Issue 2. http://www.hpastandardmethods.org.uk/pdf_sops.asp