The present invention features methods and devices for microorganisms through detecting Mie light scattering from immunoagglutinated beads. The methods feature providing a first bead suspension with antibody specific for the microorganism conjugated to beads; mixing the first bead suspension with a sample to form a first mixture; irradiating the first mixture with first incident light; detecting forward light scattering at a first angle with respect to the first incident light, where the first angle being between about 30 to 60 degrees; determining l from the light scattering; providing a second bead suspension with no antibody and simultaneously measuring l.sub.0 in a similar manner; comparing l with l.sub.0. All light scattering measurements may be made in a two-well slide or a Y-channel microfluidic device. Samples, for example food samples (e.g., vegetable samples), may be prepared in a variety of ways. A vegetable sample may be chopped up and added to a buffer. In some embodiments, the sample is then filtered with a common cloth or tissue component. The present invention also features devices (or apparatuses) for detecting a microorganism in a sample. The apparatuses may be a large-scale device or a small-scale device. The large-scale device may consist of a portable spectrometer, light source, optical fibers, and adjustable positioning stages, in addition to, for example, a two-well slide or a microfluidic device. The small-scale device is made portable by using, for example, light-emitting diodes, avalanche photodiodes, an op-amp circuit, Arduino microcontroller board, an LCD display, and small batteries, in addition to, for example, a two-well slide or a microfluidic device. Therefore, the invention is adaptable for detecting microorganisms in vegetable sample preparations. Still further, the invention may be operated on a small-scale, for example, for use by workers in agriculture fields or food factories.