|Year : 2003 | Volume
| Issue : 2 | Page : 102-107
Occurrence and characterisation of uropathogenic escherichia coli in urinary tract infections
R Raksha , H Srinivasa , RS Macaden
Department of Microbiology, St. John's Medical College, Bangalore - 560 034, Karnataka, India
Department of Microbiology, St. John's Medical College, Bangalore - 560 034, Karnataka, India
PURPOSE: To study the occurrence of Uropathogenic Escherichia coli (UPEC) in cases with urinary tract infections. METHODS: A total of 220 cases from urinary tract infections and 50 stool samples from apparently healthy individuals were included. The colonies identified as Escherichia coli were screened for virulence factors namely haemolysin, Mannose Resistant and Mannose Sensitive Haemagglutination (MRHA, MSHA), Cell surface hydrophobicity and Serum resistance by recommended methods. RESULTS: Among 220 cases 91(41.36%) were haemolytic, 68(30.9%) showed MRHA, 58(26.36%) were cell surface hydrophobicity positive and 72(32.72%) were serum resistant. In 50 controls 3(6%) were haemolytic, 6(12%) showed MRHA, 9(18%) showed cell surface hydrophobicity and 12(24%) were serum resistant. The difference between cases and controls for haemolysis and MRHA were significant (p<0.001 and p<0.01 respectively). A total of 14 atypical Escherichia coli were isolated from urine and all showed the presence of one or the other virulence markers.Out of 18 mucoid Escherichia coli isolated 10 were serum resistant. Interestingly among the 15 Escherichia coli isolated from patients with pyelonephritis 8 were UPEC. CONCLUSIONS: Out of 220 urinary isolates, 151 could be labelled as UPEC.
|How to cite this article:|
Raksha R, Srinivasa H, Macaden R S. Occurrence and characterisation of uropathogenic escherichia coli in urinary tract infections. Indian J Med Microbiol 2003;21:102-7
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Raksha R, Srinivasa H, Macaden R S. Occurrence and characterisation of uropathogenic escherichia coli in urinary tract infections. Indian J Med Microbiol [serial online] 2003 [cited 2020 Apr 9];21:102-7. Available from: http://www.ijmm.org/text.asp?2003/21/2/102/7984
Urinary tract infection is one of the most important causes of morbidity and mortality. E.coli is the most frequent urinary pathogen isolated from 50% - 90% of all uncomplicated urinary tract infections. E.coli present in the gastrointestinal tract as commensals provide the pool for initiation of UTI. It has been traditionally described that certain serotypes of E.coli were consistently associated with uropathogenicity and were designated as Uropathogenic E.coli (UPEC). These isolates express chromosomally encoded virulence markers. These markers of UPEC are expressed with different frequencies in different disease states ranging from asymptomatic bacteriuria to chronic pyelonephritis.
In the late 1970s it was recognized for the first time that E.coli strains causing urinary tract infections typically agglutinate human erythrocytes despite the presence of Mannose and this was mediated mainly by fimbriae. Subsequently an array of virulence factors have been proposed as virulence markers for uropathogenic isolates of E.coli. It is now recognized that there are a subset of faecal E.coli having the above mentioned factors which can colonise periurethral area, enter urinary tract and cause symptomatic disease. These are currently defined as UPEC.
However, most of these studies have been carried out on patient isolates and no studies have been carried out on commensal /gut isolates. In addition the relative importance of these bacterial factors has not been validated. In the present study an attempt has been made to answer these two questions with respect to four proposed virulence factors namely haemolysin, haemagglutination, cell surface hydrophobicity and serum resistance.
| ~ Materials and Methods|| |
The study was conducted in a tertiary care hospital for a period of two years from November 1998 to October 2000. Urine samples from a total of 220 cases of UTI belonging to the Departments of Urology, Nephrology, Medicine, Paediatrics and few other departments of the hospital were studied for the detection of virulence markers of E.coli.
A total of 50 stool samples from apparently healthy individuals who had come for routine health check up were screened for E.coli and studied further for virulence markers.
E.coli were identified as described by Bailey and Scott with modifications. Care was taken to collect clean catch midstream urine, which was immediately processed or if refrigerated, was processed within one hour. Considering the importance of quantitation, bacteriological loop was calibrated using Evan's blue dye solution. A battery of biochemical tests were used for the identification of E.coli. The isolates were maintained by inoculating nutrient agar butts and stored at room temperature. Standard uropathogenic E.coli serotypes O4 and O6 and E.coli ATCC 25922 were used as controls for detection of virulence markers.
All urine samples having growth of single morphotype of colony with counts 105 colonies/ml were considered significant. A count of 104-105 colonies/mL was considered “probably significant” and a count of <104 colonies/mL as insignificant growth of perineal or urethral flora.
The additional features noticed in terms of colony morphology were, whether they were mucoid/nonmucoid and biochemical reactions, whether showed hemolysis on sheep blood agar after overnight incubation, whether they were typical and atypical isolates. An isolate was considered as typical if it was a lactose fermenter and aerogenic and atypical if it was nonlactose fermenter and anaerogenic. Atypical isolates were further tested for sorbitol, cellobiose and adonitol fermentation. E.coli thus obtained from cases with significant or probably significant counts were screened for virulence markers.
Detection of Virulence Factors
| ~ Haemolysin|| |
The cytolytic protein toxin secreted by most haemolytic E.coli isolates is known as alpha haemolysin. Haemolysin was detected by determining a zone of lysis around each colony on 5% sheep blood agar plates after overnight incubation.
| ~ Haemagglutination|| |
The haemagglutination was detected by clumping of erythrocytes by fimbriae of bacteria in presence of D-mannose. The method followed was according to Siegfred et al. Bacteria grown on TSI agar medium were subcultured onto MacConkey's plates (MA) and incubated at 37°C overnight. E.coli grown on MA plates were inoculated into 5 mL of phosphate buffered saline pH 7.4 (PBS) and incubated for 5 days at 37°C to get fimbriae enriched E.coli. The pellicle formed on the surface was noted and subcultured onto colonization Factor Antigen (CFA) agar and incubated overnight at 37°C. Five millilitre of group A positive venous blood was collected using disposable syringe from a voluntary donor and added to an equal amount of Alsever's solution. This was washed three times and 3% erythrocyte suspension was made with PBS. Controls used were ATCC E.coli 25922 for mannose sensitive haemagglutination, UPEC serotypes 06 and 011 as MRHA positive controls. The procedure of Siegfred et al was modified and carried out on VDRL slides instead of microtitre plates.
Haemagglutination was considered to be mannose resistant when it occurred in presence of D-mannose and mannose sensitive when it was inhibited by D-mannose.
The slides in which isolates gave a weak reaction were placed at 4°C for five minutes and enhancement of weak reactions were checked and considered positive if obvious. Isolates which gave doubtful reactions were retested.
Cell surface Hydrophobicity
Salt aggregation test: Bacteria were tested for their hydrophobic property by using different molar concentrations of ammonium sulphate., Those which aggregated with salt particles and formed clumps were considered hydrophobic. A strain of E.coli which was haemolytic, MRHA positive and consistently positive for cell surface hydrophobicity was used as positive control. A strain which was nonlytic, negative for MRHA and consistently negative for cell surface hydrophobicity was used as negative control. E.coli grown on MA plates were inoculated into 1mL of PBS pH 6.8 and turbidity were matched with McFarland tubes 6 and 7 to get a colony count of 5x10 colonies/mL. Different molar concentrations of ammonium sulphate namely 1M, 1.4M and 2M concentrations were prepared. Forty microlitre of 0.2 M PBS pH 6.8 was taken in first column of VDRL slide. 40 µl of 1M, 1.4M and 2M concentrations of ammonium sulphate were taken in each well of other columns of VDRL slide. Forty microlitre of E.coli suspension was added to each of these wells. The clumps formed in different molar concentrations of ammonium sulphate was observed under Leights Wetzlar inverted microscope - binocular (0.32) at 20x magnification and scored positive on one to four scale (1+ to 4+). Strains were considered hydrophobic if they aggregated in concentrations of 1.4M. Isolates which gave doubtful results were retested.
Serum Bactericidal Assay
This assay was done according to Siegfried et al with modifications, such as Mueller Hinton agar plates were used in place of blood agar, and a shaker water bath was used in placed of roller angle shaker.
According to method of Benge strains were termed serum sensitive if viable count dropped to 1% of initial value and resistant if 90% of organisms survived after 180 minutes.
Chi square test was used to compare the occurrence of virulence markers in cases and controls. P value less than 0.05 was considered significant.
| ~ Results|| |
E.coliisolates from bacteriuria
Of the 220 urine samples in which E.coli was isolated, 191 had significant bacteriuria with counts 105 colonies/mL and 29 had probably significant bacteriuria with counts between 104-105 colonies/mL.
Phenotypic and biochemical characteristics of E.coli
A total of 18 Esch.coli isolates were mucoid lactose fermenting colonies. All controls were non mucoid lactose fermenters. Out of 220 isolates 14 were non lactose fermenters and anaerogenic and considered as atypical E.coli.
Virulence markers of Uropathogenic E.coli (UPEC) obtained from cases and controls
The performance of standard uropathogenic E.coli isolates were satisfactory in all the 4 assays. Occurrence of UPEC with virulence markers in different combinations among cases and controls is shown in [Table - 1].
| ~ Haemolysin|| |
Among the 220 cases 91(41.36%) were haemolytic. Among 50 controls, 3(6%) were haemolytic. The difference between cases and controls for haemolysin production was highly significant (p < 0.001).
| ~ Haemagglutination|| |
A total of 68 (30.9%) among 220 cases and 6 (12%) among 50 controls showed mannose resistant haemagglutination. There was a significant (p<0.01) difference in MRHA between cases and controls.
| ~ Cell surface Hydrophobicity|| |
A total of 58 (26.36%) among 220 cases and 9(18%) among 50 controls were cell surface hydrophobic. There was no significant difference (p > 0.05) between cases and controls, for cell surface hydrophobicity.
An E.coli isolate U56 served as a positive control and gave 1+, 3+, 4+ grade clumping in1M, 1.4M and 2M ammonium sulphate concentrations respectively. An isolate U 3055 from a case served as a negative control and gave negative results in all 3 molar concentrations.
| ~ Serum resistance|| |
Among 220 cases, 72 (32.72%) E.coli were serum resistant and among 50 controls, 12 (24%) E.coli were serum resistant. There was no significant difference (p>0.05) in cases and controls for serum resistance. The results of virulence markers among E.coli isolates from cases and controls are shown graphically in [Figure - 1] and occurrence of multiple virulence factors are shown in [Figure:2].
HLY haemolysin, HA Mannose Resistant haemagglutination, CSH Hydrophobicity, SBA Serum Resistance
UPEC and pyelonephritis
Out of 15 pyelonephritis cases eight were positive for E.coli having one or other virulence markers. Among the eight cases of uropathogenic E.coli, six were acute pyelonephritis and two were chronic pyelonephritis cases.
Atypical / Mucoid E.coli and virulence
Of the 18 mucoid lactose fermenters, 10 were serum resistant and three were haemolytic, two were MRHA, one was MSHA. Among the 14 non lactose fermenters all had one or more virulence markers.
| ~ Discussion|| |
Considering the high degree of morbidity and mortality of UTIs the subject of uropathogenic E.coli is receiving increasing attention. Cell morphology and molecular biology studies have revealed that uropathogenic E.coli express several surface structures and secrete protein molecules some of them cytotoxic, peculiar to the strains of E.coli causing UTI. Hence it is important to identify UPEC from non UPEC isolates in the urinary samples.
Results in our study showed that a large number of urinary isolates from cases had more than one virulence markers. In this case control study we conclude that UPEC strains are definitely associated with the aetio-pathogenesis of UTI.
The occurrence of multiple virulence factors in UPEC strains further strengthens the concept of association of UPEC with urinary pathogenicity. It was interesting to note that UPEC with multiple virulence factors were significantly more in cases than in controls.
Phenotypic and biochemical characterization revealed interesting findings. A total of 18 urinary isolates were mucoid and all these were obtained from cases only. No mucoid strains were isolated from control group. Further, 10 of these 18 were serum resistant. Mucoid strains are capsulated and capsule confers serum and phagocyte resistance to some E.coli strains. This property is attributed to content of sialic acid which reduces ability of bacterial surface to activate complement by alternative path way. However, 8 mucoid strains were susceptible to serum indicating non capsular factors had a role in serum resistance. Thus the conventional phenotypic marker such as capsule is also an important virulence factor for UPEC.
In the present study, 14 (6.36%) of 220 isolates were atypical E.coli. Altered phenotype could be due to an altered genetic make up. Bhat et al studied 210 E.coli strains isolated from urine and found 26 (12.4%) to be atypical. In their study, of the 26 atypical urinary isolates 12% were haemolytic and 3 had hydrophobicity (< 0.156) and were MRHA positive and all the atypical E.coli had one or other virulence marker indicating that atypical phenotype probably contributes to their virulence.
The cytolytic protein toxin secreted by most haemolytic E.coli strains is a-haemolysin. E.coli also produces cell associated lysin on blood agar plates. and haemolysin cause a clear zone of lysis. In the present study, though the nature of haemolysin was not further characterized it can be considered as cytotoxic necrotising factor ( haemolysin). The difference between cases and controls for production of haemolysin was highly significant (p< 0.001). This was similar to Johnson et al study where haemolysin was produced by 38% of urinary and 12% of faecal isolates.
Haemagglutination is mediated by fimbriae. MRHA can be mediated by P fimbriae and also X, FIC, Dr fimbriae. Thus MRHA positive strains can be considered as UPEC most likely having P fimbriae. In the present study there was a significant difference for MRHA between cases and controls (p <0.01). This was similar to a study by Johnson et al where 58% of urinary isolates and 19% of faecal isolates showed MRHA. The expression of type 1 fimbriae is indicated by MSHA. MSHA were more in faecal strains than urinary isolates in our study. More work is required to assess role of MSHA in pathogenecity.
The role of cell surface hydrophobicity (CSH) in mediating bacterial adherence to mammalian cells was conceived by Mudd and Mudd. Crystalline surface layers “S” layer present on both gram negative and gram positive organisms, play a role in this. Hydrophobicity is a recently described novel virulence mechanism by E.coli. In the present study, though there was no significant difference for CSH between cases and controls (p>0.05) more isolates from cases were hydrophobic.
Taylor reviewed that bacteria are killed by normal human serum through lytic activity of alternative complement system. Bacterial resistance to killing by serum results from individual or combined effects of capsular polysaccharide, O polysaccharide and surface proteins. Although more E.coli isolates from cases were serum resistant compared to controls the difference was not statistically significant (p>0.05). The O group designation reveals little about a strain. The apparent virulence associated with certain groups may be mediated through other virulence factors like P fimbriae, haemolysin, serum resistance, which are commonly associated with UTI associated strains. Therefore serotyping was not attempted in the present study.
In this study, eight out of 15 cases of pyelonephritis were caused by uropathogenic E.coli with one or more virulence markers. Further studies need to be done on pyelonephritonegic isolates. In this study, 151 out of 220 isolates had one or more virulence factors. When a comparison was done between urinary and faecal E.coli isolates, haemolysin production, presence of capsule and capacity to cause MRHA emerged as important virulence factors. The haemolysin, especially a-haemolysin, also known as cytotoxic necrotising factor, is strongly proinflammatory leading to secretion of IL - 6 and chemotaxins which sets pace for pathogenesis of renal disease. The capacity to cause MRHA is due to various adhesins mainly P fimbriae, P associated fimbriae, and FIC fimbriae seen in pyelonephritis cases. These adhere to fibronectin on uroepithelial cells contributing to persistence.
We believe that the methods of detection of the above mentioned virulence markers is reasonably easy and screening them in a clinical microbiology laboratory is a worthwhile exercise.
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