Disease epidemiology
Enterococcus is naturally parasitic in human and animal intestines and it is often studied as a beneficial bacterium. At present, nosocomial infection of Enterococcus is becoming more and more serious in clinic due to various factors such as irrational use of antibiotics and immune preparations, which include urinary tract infection, sepsis, endocarditis, meningitis and pneumonia, etc. [1, 2].
There are also sporadic reports of other animal infections in veterinary clinics in recent years [3]. Xinjiang in Northwest China is a major sheep-raising province with the number of sheep ranking second in China. In the past few years, lambs from some large-scale sheep farms in Northern Xinjiang have suffered from an epidemic disease with a short course, no variety difference, mainly infecting lambs aged 20–40 days, causing cerebritis symptoms, sepsis and mortality of lambs up to 20–30%. More than 60 strains(11 strains were isolated during early outbreaks, and more than 50 strains were isolated from sporadic infections later) of bacteria were isolated from the brain of sick lambs collected without contamination, which were identified as Enterococcus faecalis (E.faecalis) in Enterococcus by VITEK-AMS32 and specific PCR in the author’s laboratory [4]. The clinical symptoms and pathological changes from the lambs which were artificially infected with the isolated bacteria were similar to those naturally infected lambs [5], bacterial antigens were localized in many tissues and organs including the brain of experimental animals (unpublished). It indicated that E.faecalis was the pathogen of lamb encephalitis prevalent in some large-scale sheep farms in Xinjiang.
Organism
Enterococcus was initially isolated and identified from excrement using sodium azide, and this organism was found to be a non spore-forming Gram-positive coccus without flagella. This class of cocci was not only identified as one of the major species in the intestines of humans and animals, but is also widely distributed in mammalian gastrointestinal, respiratory, and reproductive tracts, even in soil and water. Some species of the genus have been used in the food industry.
Vancomycin-resistant Enterococci (VRE) has gradually become one of the most important pathogens of nosocomial infections since it was first isolated and reported in Duluich Hospital in 1988. The American Hospital Infection Surveillance System (NISS) has announced that Enterococcus is the second largest pathogen causing nosocomial infection [6]. So far, there were also reports about animal infections. As a representative species of Enterococcus, E. faecalis has a special and complex resistance pattern, multiple pathogenic factors and have attracted concern [7,8,9].
In recent years, a lamb infectious diseases characterized by neurological symptoms was found in northern Xinjiang, China. In the early outbreak of infection, 11 strains of E. faecalis were isolated and identified [10], more than 50 strains of E. faecalis were isolated and identified from the dead lambs (unreported) in the author’s laboratory, no other bacteria and viruses were detected. Because E. faecalis 2A (XJ05) has a wide range of drug resistance phenotypes and low LD50, it was selected for genome sequencing and made comparisons with other five E. faecalis from Genbank. The five E. faecalis can be categorized into pathogens, symbiotic bacteria, and probiotics. For example, E. faecalis V583 and E. faecalis OG1RF are pathogens, E. faecalis 62 and E. faecalis D32 are symbiotic bacteria, while E. faecalis Symbiofor 1 has been shown to have probiotic qualities [11,12,13,14,15].
Genomic research progress
In veterinary clinic, the infected lambs showed neurological symptoms, even septicemia and eventually died. Some E. faecalis were only isolated from the brains of dead lambs and no other related viruses were detected, characteristic pathological changes of encephalitis were observed. E. faecalis 2A (XJ05) with extensive drug resistance and a lower LD50 was selected from more than 60 isolates.
At present, there is a lack of systematic research about E. faecalis inducing lamb encephalitis. These E. faecalis strains were assessed using an evolutionary tree, according to mitochondria similarity. E. faecalis 2A (XJ05) and E. faecalis V583 appeared to be relatively similar, with respect to another 4 compared strains (Fig. 1).
Five E. faecalis strains have provided poor genome sequence maps in succession, including E. faecalis V583, E. faecalis D32, E. faecalis 62, E. faecalis OG1RF and E. faecalis Symbioflor 1. However, E. faecalis 2A has no detailed genome information to date.
In this study, the assembled genome map was used to study common features and specific characteristics of the strain compared with other E. faecalis strains based on genome sequencing of E. faecalis 2A (XJ05). These results may provide some clues for lamb encephalitis prevention and treatment by revealing E. faecalis virulence-associated genes and resistance mechanisms.