  Bacterial infectious diseases are a major threat to human health. Timely and
sensitive pathogenic bacteria detection is crucial in identifying the bacterial
contaminations and preventing the spread of infectious diseases. Due to
limitations of conventional bacteria detection techniques there have been
concerted research efforts towards development of new biosensors. Biosensors
offering label free, whole bacteria detection are highly desirable over those
relying on label based or pathogenic molecular components detection. The major
advantage is eliminating the additional time and cost required for labeling or
extracting the desired bacterial components. Here, we demonstrate rapid,
sensitive and label free E. coli detection utilizing interferometric
reflectance imaging enhancement allowing for visualizing individual pathogens
captured on the surface. Enabled by our ability to count individual bacteria on
a large sensor surface, we demonstrate a limit of detection of 2.2 CFU/ml from
a buffer solution with no sample preparation. To the best of our knowledge,
this high level of sensitivity for whole E. coli detection is unprecedented in
label free biosensing. The specificity of our biosensor is validated by
comparing the response to target bacteria E. coli and non target bacteria S.
aureus, K. pneumonia and P. aeruginosa. The biosensor performance in tap water
also proves that its detection capability is unaffected by the sample
complexity. Furthermore, our sensor platform provides high optical
magnification imaging and thus validation of recorded detection events as the
target bacteria based on morphological characterization. Therefore, our
sensitive and label free detection method offers new perspectives for direct
bacterial detection in real matrices and clinical samples.
