Figure 4. Growth of UTI89, F11, and MG1655 ± SAA. Bacterial growth in M9 medium with 0, 5, or 20 mg/ml SAA was determined by measuring OD600 over time. Each curve represents the means of results from quadruplicate samples. doi:10.1371/journal.pone.0032933.g004 biofilm assays. In these assays, the addition of a physiological concentration of SAA (5 ug/ml) significantly inhibited biofilm formation by UTI89 as well as F11 (Fig. 5). Discussion A few decades ago the first publication emerged indicating that SAA and CRP act as the most reliable markers for monitoring Serum Amyloid A Induction by UPEC Figure 5. Biofilm formation by the UPEC isolates UTI89 and F11 ± SAA. Biofilm levels were quantified by measuring A562 after 48 h static growth in M9 medium 60, 0.5, 1 and 5 mg/ml SAA. *, p,0.001, as determined by Student’s t test. Results show the mean values 6 SEM of three independent experiments performed in quadruplicate. doi:10.1371/journal.pone.0032933.g005 antimicrobial therapy in patients with UTIs [33]. SAA and CRP levels were also found to be higher in bacterial infections than in viral infections, although SAA appeared to be more clinically relevant as a marker of inflammation in acute viral infections [34]. Initial reports on the anti-microbial activities of SAA were published about a decade ago with work showing that recombi- nant SAA can enhance the anti-fungal activities of polymorpho- nuclear cells (PMNs) [35]. Within 30 min of stimulation, SAA was found to promote the upregulation of cytosolic Ca++ concentra- tions, cell-surface expression of CD11c and CD16 (antigens involved in adhesion and microbial recognition), and elevation of lactoferrin secretion. Lactoferrin itself is an anti-microbial agent, which enchances PMN phagocytic activity against heat-killed yeast Candida albicans. More recently, SAA was shown to interact with hepatitis C (HCV) virions, thereby blocking viral entry [6]. Although no clinical correlation in this study could be found between sera levels of SAA and HCV viral loads, a tight relationship between SAA and HDL levels in modulating HCV infectivity has been proposed [3,5]. SAA has also been shown to inhibit HIV-1 infection of host cells via CCR5 receptors [5,7]. SAA appears to be one of the earliest systemic anti-viral responses to HIV-1, being induced as early as 5–7 days prior to the first detection of plasma viral RNA and considerably earlier than other systemic cytokines. Such observations indicate that SAA can act as a front line anti-viral defense prior to systemic activation of other immune responses [5]. SAA also appears to play a major role as host defense against bacterial pathogens, acting as an opsonin able to rapidly bind with high affinity to many Gram-negative bacteria, including E. coli, Pseudomonas aeruginosa, Salmonella typhimurium, Shigella flexneri, Klebsi- ella pneumoniae, and Vibrio cholerae [4]. Binding of SAA to E. coli was found to be mediated primarily through interactions with OmpA, as OmpA deficient E. coli did not to bind SAA [9]. Importantly, the concentration of SAA required for half-maximal binding to E. coli is within physiologically relevant levels and does not require other acute phase responses [4]. In vitro, neutrophils phagocytose K-12 E. coli more readily and in higher numbers following opsonization of the bacteria with physiologically normal levels of SAA [9]. More recently, overexpression of SAA1/2 by epithelial cells has been shown to reduce the viability of a co-culured K-12 E. coli strain via an as-yet undefined mechanism [12]. In contrast to these antimicrobial effects, SAA may in some cases be detrimental to the host. For example, SAA has been shown to PLoS ONE | www.plosone.org 5 March 2012 | Volume 7 | Issue 3 | e32933