Gram-Positive Uropathogens, Polymicrobial Urinary Tract Infection, and the Emerging Microbiota of the Urinary Tract

Abstract

Gram-positive bacteria are a common cause of urinary-tract infection (UTI), particularly among individuals who are elderly, pregnant, or who have other risk factors for UTI. Here we review the epidemiology, virulence mechanisms, and host response to the most frequently isolated Gram-positive uropathogens: Staphylococcus saprophyticus, Enterococcus faecalis, and Streptococcus agalactiae. We also review several emerging, rare, misclassified, and otherwise underreported Gram-positive pathogens of the urinary tract including Aerococcus, Corynebacterium, Actinobaculum, and Gardnerella. The literature strongly suggests that urologic diseases involving Gram-positive bacteria may be easily overlooked due to limited culture-based assays typically utilized for urine in hospital microbiology laboratories. Some UTIs are polymicrobial in nature, often involving one or more Gram-positive bacteria. We herein review the risk factors and recent evidence for mechanisms of bacterial synergy in experimental models of polymicrobial UTI. Recent experimental data has demonstrated that, despite being cleared quickly from the bladder, some Gram-positive bacteria can impact pathogenic outcomes of co-infecting organisms. When taken together, the available evidence argues that Gram-positive bacteria are important uropathogens in their own right, but that some can be easily overlooked because they are missed by routine diagnostic methods. Finally, a growing body of evidence demonstrates that a surprising variety of fastidious Gram-positive bacteria may either reside in or be regularly exposed to the urinary tract and further suggests that their presence is widespread among women, as well as men. Experimental studies in this area are needed; however, there is a growing appreciation that the composition of bacteria found in the bladder could be a potentially important determinant in urologic disease, including susceptibility to UTI.

Figure 1. S. saprophyticus virulence factors

(A) Secreted surface proteins: Aas, possesses N-terminal signal sequence, but no motifs such as a transmembrane domain, LPXTG sortase recognition motif, or proline/glycine-rich cell wall-spanning domain to indicate the mode of attachment to the cell surface after translocation across the membrane (indicated by the purple question marks) [72]. Immuno-elctron microscopy shows Aas as part of a fuzzy surface layer that is absent when Aas is not expressed [75]. Ssp has a YSIRK-containing signal sequence but no sortase-recognition motif so its mode of attachment to the cell surface is uncertain (indicated by the red question mark); it is easily sheared from the cell surface. Electron microscopy and immuno-electron micrographs also show Ssp to exist as part of fuzzy surface layer, apparently consisting of 50–75nm fibrillar structures; the nature of these fibers in not known [75, 76]. (B) UafA, SdrI, SssF, and UafB contain an LPXTG motif and are predicted to be covalently attached to the cell wall [78, 79, 81]. The small arrows near the membrane-anchored sortase enzymes indicate the two-step transpeptidation reaction whereby sortase substrate are first cleaved within the LPXTG motif to create a sortase-substrate intermediate (and releasing the membrane domain and positively charged cytoplasmic tail, indicated by the straight line in the membrane) that is then resolved resulting in covalent linkage of the substrate to the cell wall [380]. UafB is genetically linked to accessory secretion genes secA2 and secY2 that are predicted to encode a dedicated accessory secretion system for UafB [79]. (C) Cytoplasmic enzymes that promote S saprophyticus survival in urine [381, 382].

Figure 2. Gram-positive inhabitants and pathogens of the human urinary tract

Approximate phylogenetic relationships between Gram-positive bacteria are illustrated in this schematic representation. Please refer to the text and Table 1 for additional information and references describing these genera as uropathogens or inhabitants of the human urinary tract. On the left, Bifidobacterium and Gardnerella belong to the order Bifidobacteriales, which together with the orders Priopionibacteriales, Actinomycetales, and Corynebacteriales belong to the class Actinobacteria (a.k.a. “high GC Gram positive bacteria”). Atopobium belongs to the order Coriobacteriales and the class Coriobacteriia. The classes Coriobacteriia and Actinobacteria both belong to the phylum Actinobacteria. The remaining genera with the exception of Mycoplasma belong to the phylum Firmicutes. Whereas Peptostreptococcus, Anerococcus, Finegoldia and Peptoniphilus belong to the order Clostridiales and the class Clostridia, Staphylococcus and Gemella belong to the order Bacilliales and the class Bacilli. Members of the order Lactobacilliales (Lactobacillus, Aerococcus, Enterococcus, and Streptococcus), are also classified as Bacilli. The genus Veillonella is also a member of the Firmicutes, but belongs to the class and order Negativicutes and Selenomonodales respectively. On the other hand, Mycoplasma belongs to the phylum Tenericutes, the class Mollicutes and the order Mycoplasmatales.

Figure 3. Transmission electron micrographs of several urogenital isolates from the phylum Actinobacteria

Strains grown for 24–48 hours underwent negative staining with uranyl acetate and were examined by TEM. These strains were isolated from the urine or vaginas of pregnant or nonpregnant women and are available through BEI resources. Strain names are as follows: Actinomyces neuii, MJR8396A; Alloscardovia omnicolens, CMW7705A; Atopobium vaginae, CMW7778A; Bifidobacterium bifidum, MJR8628B; Gardnerella vaginalis, PSS7772B; Propionibacterium avidum, MJR7694. Scale bars are 500nm. Shaded backgrounds contain images of the same strain.