When bacteria invade any part of the urinary tract, they are capable of causing UTIs [1]. Among myriads of bacterial infections, UTIs are the most common [2] [3] [4] [5]. Urinary tract infections affect all ages but its prevalence varies with age and sex [6]. Successful diagnosis depends on presence of symptoms and cultural isolation of the etiologies [2]. Women are generally more prone to UTIs than men [2] as well as patients with immunodeficiencies, urologic abnormalities and extremes of ages [6].

Escherichia coli (E. coli) is the most common cause of (80–90%) UTIs [5] [6][7]. It can be caused by Proteus mirabilis, Klebsiella pneumoniae (K. pneumoniae), Enterobacter spp., Serratia spp., Pseudomonas aeruginosa (P. aeruginosa), Staphylococcus saprophyticus and Candida spp. [6] [7] [8]. Where the anterior of the urinary tract was infected (causing urethritis), Neisseria gonorrhoeae (with predominance in men), staphylococci, streptococci and chlamydiae have been implicated [7]. Salmonella typhi and Salmonella paratyphi can be recovered from urine, especially in typhoid carriers who shed the bacteria for many years, even commonly in endemic regions of schistosomiasis [7].

The occurrence of >10⁵ significant bacteriuria in fresh or boric acid-preserved urine [7] is not a necessary indication of serious UTI in an individual [9]. Hence, such an individual may be asymptomatic with no overt pains or inflammation [10].

Antimicrobial resistance among these uropathogens is a threat for increasing economic burden [8] , which has been estimated at $1.6 billion [2] to $6 billion [11] annual cost of treatment. However, catheters-associated UTIs are the most prevalent cases of nosocomial infections [2] of 40% due to formation of biofilm [11]. Urinary tract infections are divided into two: lower UTIs (including cystitis, urethritis, prostatitis), and upper UTI (including pyelonephritis, intra-renal abscess, perinephric abscess) [5] [6].

UTIs can either be complicated or uncomplicated [6]. Men <50 years have rare cases of bacteriuria but UTIs increase in men above that age. Women are more frequently affected by UTIs [5] [7]because of the shortness of their urethras; they remain prone to both asymptomatic and symptomatic UTIs during pregnancy [7]. About one in every three women by the age of 24 will have developed UTI [2] and many of the uncomplicated cases result in long-term sequalae or renal damage [12]. Over 150 million global cases of UTIs occur annually [12]. Generally, half of all women will suffer from at least one symptomatic UTI in their lifetime [2][12] but only 20% of all cases occur in men [12]. Uropathogens get into the urinary tract via hematogenous or ascending routes [5] [6]. A greater threat can be posed by undetected/untreated persistent or recurrent asymptomatic UTIs, which can lead to pyelonephritis and renal failures [7] [12] but they are not common [13]. Other bacteria are becoming emerging etiologies of UTIs, for example Mycoplasma genitalium [11].

In view of complications, community-acquired UTIs are often uncomplicated but almost all nosocomial UTIs are not only complicated but serve as the largest hospital reservoir of nosocomial antibiotic-resistant uropathogens [14]. It is important to correctly diagnose UTI and make appropriate choice of antibiotics for treatment.

Study area
The research was conducted in Ahmadu Bello University (A.B.U.), Samaru Campus Zaria, Nigeria. The subjects were students (between 15–45 years) of the University that presented at the University Health Services (Sick-Bay) with urinary tract complaints. The University is located on latitude 11°15'N to 11°3'N and longitude 7°30'E to 7°45'E [15].

Urine sample collection
All students with urinary tract complaints were asked and guided on how submit 5 ml early morning MSU in sterile wide-mouth, screw-capped plastic sampling bottles as suggested by Cheesbrough [7]. The samples were transferred immediately for analysis at Bacteriology Laboratory in the Department of Microbiology, A.B.U. Zaria, Nigeria.

Cultural isolation of etiologies
Each of the samples was mixed by gently shaking before opening the cap. Aseptically, inoculations were made on prepared sterile plates of CLED agar [7] and MSA. All inoculated plates were incubated aerobically at 37°C for 24–48 hr and observations were taken thereafter. Distinct colonies were Gram-stained. Pure isolates were maintained on Nutrient Agar (NA) slants at 4°C for further laboratory investigations. Biochemical tests (including catalase, coagulase, indole, Methyl red (MR), Voges Proskauer (VP), citrate utilization and sugar fermentation in Triple sugar Iron (TSI) agar) were performed. All media were prepared using instruction manuals of their manufacturers, with isolation and biochemical tests procedures as suggested by Cheesbrough [7].

Antibiotic susceptibility testing
All the isolates were subjected to the following antibiotics: ciprofloxacin (10 µg), amoxicillin (30 µg), gentamycin (10 µg), pefloxacin (30 µg), and streptomycin (30 µg), chloramphenicol (30 µg) and oflaxacin (10 µg). Inocula were prepared according to 0.5 McFarland standards. Agar disc diffusion method was used and the zones of inhibition of the UTI isolates were measured to the nearest millimeter and compared with Clinical Laboratory Standard Institute standards [16] to determine their susceptibility patterns.

Statistical analysis
Demographic data collected during MSU samples collection together with laboratory findings were subjected to chi square (χ2) and lLikelihood ratio analyses with the IBM SPSS Statistics Version 21 at p=0.05. Results were simplified in tables and charts.

Out of 100 students (comprising of 57 females and 43 males) who presented with urinary tract complaints and submitted MSU samples, only 50 samples yielded bacterial growths after a 24–48 hr aerobic incubation at 37°C. Out of these positive-culture samples, 43 (86.0%) were Gram-negative bacteria and the remaining 7 (14.0%) were Gram-positive bacteria (Figure 1).

No bacterial co-infection was found. The most prevalent bacteria were E. coli (39.0%), followed by Staphylococcus aureus (S. aureus) (7%). P. aeruginosa (1.0%) was the least occurring UTI-etiology (Table 1). In both sexes, E. coli was the most occurring UTI-etiology. About 47.4% and 53.5% of female and male MSU samples respectively were culture positive, yielding higher occurrence of E. coli and K. pneumoniae amongst the females. There was no occurrence of P. aeruginosa as UTI-etiology in males. The male students had higher occurrence of S. aureus in their MSU samples (Table 2).

Age-group 26–30 years had the highest culture-positive MSU samples (Figure 2). In the same age-group, K. pneumoniae and P. aeruginosa were absent. Also, age-group 41–45 years coincided with the highest occurrences of E. coli and S. aureus. The occurrence of E. coli increased and fell continuously with successive age-groups (Table 3). Generally, the prevalence of bacterial UTI cases gradually increased with age of the students, peaked again at age-group 26–30 years, then decreased continuously in similar fashion (Figure 2).

E. coli showed highest susceptibility to ciproflaxicin (10 µg) followed by streptomycin (30 µg). Only about half of the isolates were susceptible to amoxicillin (30 µg) and gentamycin (10 µg). Out of the three K. pneumoniae isolates, none was susceptible to gentamycin (10 µg) but all were susceptible to ciprofloxacin (10 µg). The single P. aeruginosa isolate was susceptible to chloramphenicol (30 µg), ciprofloxacin (10 µg), pefloxacin (30 µg) and oflaxacin (10 µg), but it was not susceptible to amoxicillin (30 µg) and gentamycin (10 µg). Generally, all the isolates were most susceptible to ciprofloxacin (10 µg), oflaxacin (10 µg) followed by pefloxacin (30 µg) and chloramphenicol (30 µg). Gentamycin (10 µg) had the least activity against the isolates (Table 4).

Gram-negative bacteria are among the most predominant uropathogens of UTIs. This research uncovered a frequency of 43 (86.0%) of UTI cases caused by Gram-negative bacteria. Although in 90% of reported UTI cases from Nigeria, E. coli, Proteus spp. and Klebsiella spp. are most incriminated [17]. This research also incriminated S. aureus and P. aeruginosa with exception of Proteus spp. Of these Gram-negative bacteria, E. coli was the most predominant UTI-etiology with a prevalence of 39.0%. E. coli has been implicated by many studies to be the most common cause of bacterial UTIs [6]. E. coli has a rapid adaptation and colonization of the urinary tract. There had been a reported prevalence of 40.7% of E. coli, which had occurred less than P. aeruginosa and S. aureus [17]. Gram-positive bacteria occur in UTI cases but less frequently. Staphylococcus saprophyticus (S. saprophyticus) is found among young and sexually active women [6]. Although, S. saprophyticus was not isolated in this research, S. aureus tend to occur higher in the males than females and in the age-group 41–45 years. This age group is among the sexually active groups.

Urinary tract infections are causable by many different types of uropathogens. Bacterial cases examined in this research revealed a gradual pattern of increase with increase in age of the students. It peaked in age-group 26–30 years, which is among the sexually active individuals (though information of sexual activities was not captured). Among this age group, E. coli appeared to be the most prevalent, signifying it is important in the causation of UTIs. The implication of bacterial cases of UTI among these students could indicate the possible persistence of poor personal hygiene [17].

With the exception of S. aureus (as the only Gram-positive bacterial pathogen in this research), the Gram-negative bacterial isolates were more predominant in the female students. Invariably, females are general more prone to bacterial invasion and colonization of their vaginal and urethral openings, resulting in high frequency in urinary complaints and bacteria isolation in their MSU samples. The location and structure of the female urethral and vaginal openings are far less effective in the prevention of bacterial entry [18]. Unhygienic manner of wiping the anus from back to front helps in inoculating the vulva and vagina with fecal-borne pathogens. Pregnancy and sexual intercourse have been emphasized by [19] to increase risk of UTIs in females as bacteria are pushed into the vagina during sex. Glycosuria due to increased plasma volume and decreased urine concentration favors bacterial growth in urine [19]. During vaginal childbirth, bacteria can also be push up into the bladder [20]. Hence, females are at continuous risk of ascending UTIs with further possible complications [21][22] like renal damage [12].

All the isolates were most susceptible to ciprofloxacin (10 µg) and pefloxacin (30 µg). These antibiotics are relatively expensive and hence less abused as outlined by [17]. However, gentamycin (10 µg) which is intravenously administered (and relatively cheaper) showed lesser activities against the isolates; hence, opposed the findings of [17] .

Urinary tract infections, whether complicated or uncomplicated require immediate treatment. Treatment success is a measure of adequate diagnosis and selection of recommended antibiotics. Half of the MSU samples indicated that urinary complaints among the university students were due to bacterial uropathogens. Gram-negative bacteria (86.0%) are the most prevalent bacterial uropathogens. The frequency of the bacterial uropathogens increased with increase in age, peaked at 26–30 years age-group, and then began to decrease continuously. E. coli (39.0%) remains one of the leading UTI-etiologies, which was more prevalent among the male students and in people within age-group 41–45 years. S. aureus was frequently isolated among the males. All the demographic distributions of the bacterial uropathogens were statistically insignificant (as p>0.05). All the isolated UTI-etiologies were most susceptible to ciprofloxacin (10 µg) and pefloxacin (30 µg) but gentamycin (10 µg) was ineffective.

We express our appreciation to the management and workers of Microbiology Laboratory of the University Health Services (Sick-Bay), Ahmadu Bello University, Zaria, Nigeria.

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