Detecting and counting bacteria suspended in biological fluids

Abstract

System and method for detecting and counting bacteria suspended in a biological fluid by means of light scattering measurements is provided. In accordance with the method of the invention the level of signal to noise of the measured intensities of light scattered by a sample of the biological fluid is significantly enhanced for forwardly scattered light within a range of scattering angles which are smaller compared to a predefined maximal scattering angle. The system of the invention includes a cuvette adapted to contain a sample of the biological fluid whose sidewalls and windows are suitably constructed and arranged to significantly reduce the level of reflected light obscuring the scattering patterns measured within the range of scattering angles considered.

1 . A method for detecting bacteria suspended in a biological fluid comprising 
 a. spatially filtering a light beam having an axis for illuminating a portion of said biological fluid;    b. measuring the intensity of light scattered by said biological fluid at least at one point;    c. comparing said measured intensities to a calibration scale, and    wherein said at least one point is spaced apart from said axis, and    wherein said at least one point is associated with a scattering angle not exceeding a predefined maximal scattering angle.    
   
   
       2 . A method as in  claim 1 , wherein said maximal angle does not exceed 5 degrees.  
   
   
       3 . A method as in  claim 1 , wherein said spatially filtering is accomplished by means of an aperture, which is the first aperture, wherein said first aperture has a predefined diameter.  
   
   
       4 . A method as in  claim 2 , further comprising positioning a second aperture spaced apart from said first aperture by a predefined distance.  
   
   
       5 . A method as in  claim 3 , wherein the diameter of said second aperture complies with a zero of a Bessel function of the first kind associated with a beam of light emerging off said first aperture.  
   
   
       6 . A method as in  claim 1  further comprising measuring the intensity of light scattered by said biological fluid at a second point spaced apart from said axis, wherein said second point is different than said at least one point.  
   
   
       7 . A method as in  claim 5  further comprising associating a scattering profile with said measured intensities.  
   
   
       8 . A method as in  claim 6 , further comprising matching to said associated scattering profile any item selected from a group of items consisting of pre-stored calibrated scattering profiles and linear combinations of pre-stored calibrated scattering profiles.  
   
   
       9 . A method as in  claim 1 , further comprising 
 I. successively measuring said intensity of scattered light at least twice, wherein said measuring is accomplished at a rate not lower compared to a predefined minimal repetition rate, and    II. associating a difference plot with said successively measured intensities, and    wherein said successively measuring is accomplished at the said at least one point.    
   
   
       10 . A method as in  claim 8 , further comprising 
 A. measuring the intensity of light scattered by said biological fluid at a second point spaced apart from said axis, wherein said second point is different than said at least one point;    B. associating scattering profile to said difference plot, and    C. matching to said associated scattering profile any item selected from a group of items consisting of pre-stored calibrated scattering profiles and linear combinations of pre-stored calibrated scattering profiles.    
   
   
       11 . A system for detecting bacteria suspended in a biological fluid having a light source and a light detector, said system comprising a cuvette having windows adapted to contain a sample of said biological fluid, wherein said cuvette comprises a skewed sidewall.  
   
   
       12 . A system as in  claim 11 , further comprising a spatial filter disposed adjacent to said light source, wherein said spatial filter comprises 
 a first aperture disposed adjacent to said light source;    a second aperture coaxial with the first aperture spaced apart from said first aperture by a predefined distance, and    wherein one of said windows, which is the first of said windows, is inclined relative to the normal of said axis by a first predefined angle, and wherein the second of said windows is inclined relative to the normal of said axis by a second predefined angle which is different than the first predefined angle, and wherein said first predefined angle and said second predefined angles correspond to rotations at the same direction.    
   
   
       13 . A system as in  claim 12 , wherein the diameter of said second aperture complies with a zero of the Bessel function of the first kind associated with a beam of light emerging off said first aperture.  
   
   
       14 . A system as in  claim 12 , further comprising a light obscuring means for shading a segment across the face of said light detector.  
   
   
       15 . A system as in  claim 12 , further comprising a detecting segment disposed across a surface of said light detector, wherein said detecting segment associated with a maximal scattering angle.  
   
   
       16 . A system as in  claim 12 , wherein the magnitude of said first predefined angle is in the range of 2°-15°.  
   
   
       17 . A system as in  claim 16 , wherein the second predefined angle is larger compared to the magnitude of said first predefined angle.  
   
   
       18 . A system as in  claim 16 , wherein the magnitude of the second predefined angle is larger by 8° compared to the magnitude of said first predefined angle.