A SHORT NOTE ON Gardnerella vaginalis

Gardnerella vaginalis    

Compiled by: Zenisha Acharya 

                        Tulasi Chamling Rai

Central Department of Microbiology, Kirtipur

INTRODUCTION

•       Gardnerella vaginalis is Gram negative or Gram variable bacillus.

•       It is facultative anaerobe.

•       G. vagnalis is colonizer in vagina of 40% of normal woman.

•       However, it is found to be mainly associated with bacterial vaginosis (BV) and is not considered as normal vaginal flora.  

•       Bacterial vaginosis (BV) is a clinical symptomatic condition where there is malodorous vaginal discharge.

•       Formerly, it was termed as non-bacterial vaginitis.

•       BV occurs in women of child-bearing years

•       Other BV causing organism are Prevotella, Mobiluncus. 

HISTORY

•       G. vaginalis was first described by Gardner and Dukes in 1955 and was named Haemophilus vaginalis.

•       But its growth didn’t require neither X nor V factor.

•       Later on, Zinneman and Turner(1963) classified it on genus Corynebacterium as some strains were observed as Gram positive rod.

•       Greenwood and Pickett(1980) finally proposed its removal to new genus, Gardnerella based on its cell wall composition, electron microscopic appearance and (G+C) content.  

•       G. vaginalis is the only species under genus Gardnerella.

 

CLASSIFICATION

Domain                 Bacteria

              Phylum               Actinobacteria

              Class                   Actinobacteria

              Order                  Bifidobacteriales

              Family                Bifidobacteriaceae

              Genus                 Gardnerella

              Species               Gardnerella vaginalis      

MORPHOLOGY

•       G. vaginalis is Gram negative or Gram variable rod with dimension 1.5-2.5µm X 0.5µm.

•       It is facultatively anaerobic in nature.

•       It is non-spore forming and non-motile bacilli. 

CULTURAL AND BIOCHEMICAL   CHARACTERISTICS

•       G. vaginalis doesn’t grow on nutrient agar(NA) but on chocolate agar(CA) and blood agar(BA).   

•       Growth is more prominent on human and rabbit blood agar rather than sheep and horse blood agar.

•       The bacterial growth occurs between 25  ͦC-40  ͦC while optimum at 35  ͦC- 37  ͦC.

•       Increased  carbon-dioxide  enhances growth that maintaining anaerobic environment. 

•       Colonies on CA is pin point after 24 hrs of incubation.

•       After 48 hrs of incubation colonies measures 0.5mm in diameter, circular, convex and grey colored.  

•       On BA, the colonies are β-haemolytic.

•       Characteristic β-haemolysis is more clear on two layer plates with Columbia agar alone as a base and a top layer of Columbia agar containing 5% outdated banked human blood.

•       Tween 80 and 1% proteose peptone enhance the haemolysis.

•       G. vaginalis utilizes various sugar like fructose, galactose, mannose, ribose and starch with acid production.

•       But variable results are obtained from lactose, sucrose and xylose.

•       Hence, carbohydrate fermentation test is not considered to be reliable test.

•       It is negative for catalase, oxidase, indole, urease test and nitrate reduction test.

•       H₂S gas production is also negative.

•       Organism hydrolyses hippurate.

                                           PATHOGENESIS

•       G. vaginalis is acquired during sexual activities.        

•       Invasion is initiated by the adherence of organism on the squamous epithelial cells of vagina.

•       Organism then produces, vaginolysin a cholesterol dependent cytolysin which is specific to host receptor site.

•       It encodes a pore forming toxin that binds to the CD59 human complementary regulatory  molecule.

•       This cytotoxin aid in the initial adherence.   

•       Production of biofilm further assist in the successful colonization which is achieved through sialidase enzyme. 

•       Moreover, G. vaginalis shows symbiosis relation with other anaerobes like Atopobium, Prevotella and Mobiluncus. 

•       Prevotella like species synthesizes exogenous protein.

•       G. vaginalis causes proteolysis of these exogenous protein for energy.

•       This in turn causes increment in ph value of vaginal environment.

•       On the other hand, G. vaginalis shows antagonism with vaginal normal flora Lactobacillus.

•       Antagonistic relation is maintained by the production of bacteriocin by G. vaginalis that inhibit the growth of Lactobacilli.        

•       Finally,  these all series of events result in the subsiding of Lactobacilli with the successful establishment and flourishment of BV causing G. vaginalis.

MEDICAL IMPORTANCE

•       BV is more likely to occur among women who have sex with women(WSW) and person having multiple sex partners.

•       BV is characterised with abnormal vaginal discharge which is fishy in odour, increased Ph and clue cells.

•       Clue cells are actually the sloughing of vaginal epithelial cells.

•       It is visualized as squamous epithelial cells surrounded by Gram variable rod shaped bacterium.

 

                Fig: Clue cells (squamous epithelial cells surrounded by Gram variable bacterium)

•       Its complication may lead to urinary tract infection, amnionitis, preterm birth and post-caesarean endometritis.

•       Condition is worsen during the menstrual period.

•       In men, balanoposthitis is found to occur rarely. 

•       It remains asymptomatic in male urethra and may serve as carrier but still its epidemiology is unclear.

LABORATORY DIAGNOSIS

•       Organism can be isolated from vaginal discharge. Other samples may include blood, urine and pharynx.

•       It grows well on CA and BA with β-haemolytic colonies supported with 5-10% CO₂.

•       Columbia agar contaning colistin and nalidix acid is prefered more.

•       On performing mnicroscopy, clue cells is significant observation for BV caused by G. vaginalis.

•       Organism is catalse and oxidase test negative.

•       Also, negative for urease, indole test and nitrate reduction test.

•       Hippurate hydrolysis is positive.

•       Along with these test, production of ammonical fishy smell when 10% sodium hydroxide is added to vaginal discharge confirm G. vaginalis. 

•       PCR can be done to detect genes encoding 60kDa heat-shock protein chaperonin (Cpn 60).

TREATMENT

•       G. vaginalis is susceptible to penicillin, ampicillin, vancomycin and clindamycin.

•       Nearly all strains are inhibited by metronidazole 128mg/l.